Hydraulic Oil Compatibility Chart

3. Hydraulic Oil Types and Base Stocks

Hydraulic fluids can be classified according to their base stock, additive system, fire resistance, biodegradability, and performance level. Identifying the fluid family is the first step in determining whether two oils can be mixed or whether a complete system flush is required.

Mineral-Based Hydraulic Oils

Mineral hydraulic oils are refined from petroleum and are the most common fluids used in industrial and mobile hydraulic systems. They offer good lubrication, broad equipment compatibility, and relatively low cost.

Common mineral-oil classifications include:

• H: Uninhibited mineral oil
• HL: Mineral oil with oxidation and rust inhibitors
• HM: HL fluid with additional anti-wear properties
• HV: HM fluid with an improved viscosity index
• HLP: Anti-wear hydraulic oil classified under DIN 51524-2
• HVLP: High-viscosity-index anti-wear oil classified under DIN 51524-3

Mineral oils are generally compatible with NBR, HNBR, FKM, and many polyurethane seals, but the actual operating temperature and additive package must also be considered.

Synthetic Hydrocarbon Fluids

Synthetic hydrocarbons, particularly polyalphaolefins or PAOs, are engineered to provide better oxidation resistance, low-temperature fluidity, and high-temperature stability than conventional mineral oil.

PAO fluids may be broadly compatible with many mineral oils because both are hydrocarbon-based. However, differences in additives, seal conditioners, viscosity, and solvency can still make direct mixing undesirable.

Synthetic Ester Fluids

Synthetic ester fluids are used in applications requiring biodegradability, high-temperature performance, or improved fire resistance. They provide excellent lubrication but may interact differently with seals, coatings, residual mineral oil, and moisture.

Synthetic esters may hydrolyze when exposed to excessive water and heat. Compatibility with NBR, FKM, polyurethane, paints, and adhesives should be confirmed before conversion.

Natural Ester Fluids

Natural ester hydraulic fluids are produced from vegetable oils such as rapeseed, sunflower, or soybean oil. They are commonly used where leakage could affect soil, water, forestry, agriculture, or other environmentally sensitive areas.

Natural esters offer:

• High biodegradability
• Good lubricity
• High viscosity index
• Lower environmental impact

However, they may have lower oxidation stability than premium synthetic fluids and can be sensitive to water, high temperatures, and contamination with conventional mineral oil.

Polyalkylene Glycol Fluids

Polyalkylene glycol, or PAG, fluids are synthetic lubricants known for excellent lubricity, high viscosity index, low deposit formation, and resistance to varnish.

PAG fluids are normally not compatible with mineral oil, PAO, or most ester fluids. Even relatively small amounts of residual oil can produce cloudiness, phase separation, or performance loss. A thorough flushing procedure is therefore normally required when converting to or from a PAG fluid.

Some PAG fluids are water-soluble, while others are water-insoluble. These two PAG groups should not automatically be assumed to be mutually compatible.

Fire-Resistant Hydraulic Fluids

Fire-resistant fluids are used near furnaces, casting machines, mines, power plants, and other locations where leaking oil could contact an ignition source.

Common fire-resistant fluid groups include:

• HFA: Oil-in-water emulsions or high-water-content fluids
• HFB: Water-in-oil emulsions
• HFC: Water-glycol fluids
• HFDR: Water-free phosphate ester fluids
• HFDU: Water-free fire-resistant fluids based on other chemistries, often synthetic or natural esters

These fluids differ substantially from conventional mineral oils. Changing between mineral oil and a fire-resistant fluid generally requires checking every seal, hose, coating, pump, and filter for compatibility.

Biodegradable Hydraulic Fluids

Environmentally acceptable hydraulic fluids are often classified according to their base stock:

• HETG: Triglyceride-based fluids, usually vegetable oils
• HEES: Synthetic ester fluids
• HEPG: Polyglycol-based fluids
• HEPR: Synthetic hydrocarbon fluids, commonly PAO-based

The term biodegradable does not identify a single chemistry. Two biodegradable products may have completely different base stocks and may not be compatible with each other.

4. Hydraulic Oil Compatibility Chart

The following chart provides general guidance for mixing the main hydraulic fluid families. It applies only as a preliminary screening tool. Compatibility can vary by manufacturer, formulation, additive system, viscosity grade, fluid age, and contamination level.

*Compatibility should be confirmed by the fluid manufacturer or through laboratory testing.

Meaning of the Compatibility Ratings

Generally compatible means that fluids within the same family are unlikely to produce an immediate chemical reaction. It does not guarantee that the mixture will retain its original viscosity, anti-wear protection, demulsibility, filterability, or equipment approvals.

Usually compatible means the base stocks are often miscible, but the additive packages and performance properties must still be evaluated.

Limited or conditional compatibility means small residual quantities may sometimes be tolerated during a carefully controlled conversion. Intentional mixing is not recommended without supplier approval and testing.

Incompatible means the fluids should not be mixed. A complete drain, cleaning, flushing, seal review, and filter replacement may be required.

Important Limitations of the Chart

Fluid compatibility should be evaluated at the expected mixing ratio. A mixture containing 1% residual fluid may behave differently from a 50:50 mixture. When laboratory testing is performed, common ratios include:

• 95% new fluid and 5% existing fluid
• 90% new fluid and 10% existing fluid
• 50% new fluid and 50% existing fluid
• 10% new fluid and 90% existing fluid

Tests may evaluate:

• Visual clarity and phase separation
• Sediment and sludge formation
• Viscosity changes
• Foaming and air release
• Filterability
• Water separation
• Acid number
• Corrosion protection
• Seal response
• Storage stability at high and low temperatures

The absence of visible sludge does not prove full compatibility. The mixture may still have inadequate wear protection, poor air release, or reduced oxidation stability.

5. Mineral Oil Compatibility: H, HL, HM, HV, and HLP Fluids

Mineral-based hydraulic oils share a petroleum-derived base stock, but their additive systems and performance levels are not identical. Understanding these classifications helps prevent the replacement of a high-performance fluid with a product that provides insufficient protection.

H Hydraulic Oil

H fluid is essentially mineral oil without significant performance-enhancing additives. It offers basic lubrication but provides limited protection against oxidation, corrosion, wear, and deposit formation.

H oils are rarely selected for demanding modern hydraulic equipment. Mixing an H oil into a more highly formulated fluid can dilute the additive concentration and reduce the performance of the final blend.

HL Hydraulic Oil

HL fluid contains additives that improve:

• Oxidation resistance
• Rust prevention
• Corrosion protection
• Fluid service life

HL oil is suitable for systems that require inhibited hydraulic oil but do not need strong anti-wear performance. Although it is generally miscible with HM or HLP mineral oil, mixing can lower the anti-wear protection of the resulting blend.

HM Hydraulic Oil

HM fluid includes the oxidation and corrosion protection of an HL oil plus anti-wear additives. It is commonly used in systems containing vane, piston, or gear pumps operating under moderate to demanding conditions.

HM oils are often marketed as anti-wear hydraulic oils. They may use zinc-containing or ashless anti-wear chemistry. Oils using these different technologies should not automatically be mixed, even when both products meet the same general performance classification.

HV Hydraulic Oil

HV fluid provides the anti-wear performance of an HM oil together with improved viscosity-temperature behavior. Viscosity-index improvers help the oil remain less viscous during cold starts and maintain adequate viscosity at elevated operating temperatures.

HV fluids are frequently selected for:

• Mobile machinery
• Outdoor equipment
• Systems exposed to seasonal temperature changes
• Applications with a wide operating-temperature range

Mixing HV oil with HM, HL, or ordinary HLP oil may dilute its viscosity-index improver and reduce its cold-start or high-temperature performance.

HLP Hydraulic Oil

HLP is a common DIN designation for mineral-based hydraulic oil containing oxidation, corrosion, and anti-wear additives. Its general performance purpose is similar to that of an ISO HM fluid, although the classifications come from different standard systems and should not be treated as perfectly identical without checking the product data sheet.

Related DIN classifications include:

• HL: Oxidation- and corrosion-inhibited hydraulic oil
• HLP: Anti-wear hydraulic oil
• HVLP: HLP-type oil with improved viscosity-temperature behavior
• HLPD: Detergent and dispersant hydraulic oil

HLPD fluids deserve particular caution. Their detergent and dispersant properties allow them to hold water and contaminants in suspension. Mixing an HLPD oil with a conventional demulsifying HLP fluid may impair water separation and create unpredictable contamination behavior.

General Mineral Oil Compatibility Chart

*Confirm additive compatibility, viscosity grade, OEM requirements, and supplier approval before mixing.

Can Different Viscosity Grades Be Mixed?

Mineral oils with different ISO viscosity grades are often physically miscible, but the blend will have a viscosity between those of the two original oils. The exact result is not always a simple arithmetic average.

For example, mixing ISO VG 32 with ISO VG 68 may produce a blend near an intermediate viscosity, but the final oil may not consistently satisfy ISO VG 46 limits or the equipment manufacturer’s requirements.

Mixing viscosity grades can affect:

• Cold-start performance
• Lubricating film thickness
• Internal leakage
• Pump efficiency
• Heat generation
• Valve response
• Viscosity index

Different viscosity grades should therefore not be mixed as an informal method of creating another grade.

Recommended Practice for Mineral Oil Mixing

Before combining two mineral hydraulic oils:

1. Confirm that both products use compatible base stocks.
2. Compare their ISO or DIN performance classifications.
3. Check whether the oils use zinc-containing or ashless additive systems.
4. Confirm the required ISO viscosity grade.
5. Review water-separation and detergent characteristics.
6. Obtain written guidance from the oil supplier.
7. Conduct compatibility testing when system reliability is critical.
8. Drain and flush the system if compatibility cannot be established.

Even when two mineral oils are described as compatible, keeping the system filled with a single approved product provides more predictable lubrication, filtration, oil-analysis results, and maintenance performance.

6. Compatibility of Synthetic and Fire-Resistant Hydraulic Fluids

Synthetic and fire-resistant hydraulic fluids require more careful compatibility evaluation than conventional mineral oils. These products may use completely different base stocks, additive systems, and water contents. A fluid that is suitable for one hydraulic system may damage the seals, hoses, coatings, or components in another.

Changing from mineral oil to a synthetic or fire-resistant fluid should therefore be treated as a complete fluid-conversion project rather than a routine oil change.

PAO Synthetic Hydraulic Fluids

Polyalphaolefin fluids are synthetic hydrocarbons with a chemical structure similar to highly refined mineral oil. They offer:

• High oxidation resistance
• Good low-temperature fluidity
• High viscosity index
• Low volatility
• Extended service life

PAO fluids are generally miscible with mineral oils, and both fluid types are often compatible with similar sealing materials. However, miscibility does not guarantee equivalent system performance.

Mixing PAO and mineral oil may:

• Reduce the oxidation stability of the PAO
• Shorten the expected drain interval
• Change low-temperature properties
• Dilute anti-wear additives
• Affect seal swell
• Eliminate OEM or supplier approval

A small amount of residual mineral oil may be acceptable during an approved conversion, but deliberate mixing is generally not recommended.

Synthetic Ester Hydraulic Fluids

Synthetic esters are commonly used in biodegradable, high-temperature, and fire-resistant hydraulic fluids. They provide excellent lubricity and usually have a high viscosity index.

Their main compatibility concerns include:

• Sensitivity to moisture and hydrolysis
• Interaction with certain elastomers
• Softening or lifting of some paints and coatings
• Dissolution of deposits left by mineral oils
• Possible incompatibility between different ester formulations

Synthetic esters can have greater solvency than mineral oil. After conversion, they may loosen varnish and deposits from reservoirs, piping, and components. These materials can rapidly load filters even when the new fluid is chemically compatible with the old fluid.

Mineral oil contamination may also reduce the biodegradability, fire resistance, and environmental classification of an ester fluid.

Natural Ester Hydraulic Fluids

Natural esters are primarily produced from vegetable oils. They offer excellent lubricity, high viscosity index, and good biodegradability but can be more sensitive to oxidation than premium synthetic esters.

Natural ester fluids should not automatically be mixed with:

• Mineral hydraulic oil
• PAO fluids
• Synthetic esters
• PAG fluids
• Water-glycol fluids
• Phosphate esters

Small amounts of mineral oil may sometimes be tolerated during conversion, but they can reduce biodegradability and alter oxidation stability. The acceptable residual concentration must be obtained from the fluid supplier.

Polyalkylene Glycol Fluids

PAG fluids may be water-soluble or water-insoluble, depending on their chemical structure. They offer strong resistance to varnish formation and can provide excellent high-temperature lubrication.

PAG fluids are generally incompatible with:

• Mineral oil
• PAO
• Natural ester
• Synthetic ester
• Phosphate ester
• Conventional anti-wear hydraulic oils

Mixing PAG with a hydrocarbon-based fluid may cause:

• Cloudiness
• Phase separation
• Gel formation
• Filter blockage
• Additive precipitation
• Loss of lubricating performance

Water-soluble and water-insoluble PAG fluids may also be incompatible with each other. Conversion to a PAG fluid normally requires extensive draining, cleaning, flushing, and verification of the remaining concentration of the previous oil.

Water-Glycol Fluids

Water-glycol fluids are usually classified as HFC fire-resistant hydraulic fluids. They commonly contain water, glycol, corrosion inhibitors, anti-wear agents, and other additives.

Their fire resistance depends heavily on maintaining the correct water content. Water loss through evaporation can increase viscosity and reduce fire-resistant performance.

HFC fluids should not be mixed with:

• Mineral hydraulic oils
• PAO fluids
• Ester fluids
• Phosphate esters
• Water-in-oil emulsions
• Unapproved water-glycol products

Mineral oil contamination can create deposits, destabilize the fluid, affect seals, and impair fire resistance. Different HFC products may also use incompatible thickener and additive technologies, so fluids from different manufacturers should not be mixed without approval.

HFA High-Water-Content Fluids

HFA fluids contain a very high proportion of water. Depending on their formulation, they may be supplied as:

• Oil-in-water emulsions
• Synthetic concentrates dissolved in water
• Ready-to-use high-water-content fluids

They provide excellent fire resistance but relatively limited lubrication compared with oil-based fluids. They require equipment specifically designed for high-water-content operation.

HFA fluids are normally incompatible with mineral oil and other hydraulic fluid families. Mixing different HFA concentrates can also destabilize the emulsion or reduce corrosion protection.

HFB Water-in-Oil Emulsions

HFB fluids consist of water dispersed in a continuous oil phase. They combine some mineral-oil lubrication characteristics with improved fire resistance.

They are sensitive to:

• Incorrect water concentration
• Contamination with other oils
• Excessive temperature
• Microbial contamination
• Mixing with unrelated emulsion formulations

Adding conventional hydraulic oil can change the water-to-oil ratio and reduce fire resistance. Adding an incompatible water-based fluid can cause emulsion breakdown or phase separation.

Phosphate Ester Fluids

Phosphate ester fluids, commonly classified as HFDR, are water-free synthetic fire-resistant fluids. They are used in turbines, steel mills, aviation equipment, die-casting systems, and other high-fire-risk applications.

Phosphate esters are generally incompatible with:

• Mineral oils
• PAO fluids
• PAG fluids
• Natural and synthetic esters
• Water-glycol fluids
• Conventional NBR sealing systems

They can also attack certain paints, adhesives, plastics, hose materials, and elastomers. Systems designed for phosphate ester service often use specially selected seals, coatings, and components.

Even minor contamination by mineral oil may reduce fire resistance and alter viscosity. Conversion to phosphate ester service therefore requires a rigorous cleaning and flushing process.

HFDU Water-Free Fire-Resistant Fluids

HFDU is a broad category covering water-free fire-resistant fluids that are not phosphate esters. Products may be based on:

• Synthetic esters
• Natural esters
• PAG chemistry
• Other specialty synthetic fluids

Because HFDU describes a performance category rather than one specific chemistry, two HFDU fluids are not necessarily compatible. Their base stocks must be identified before compatibility can be assessed.

Synthetic and Fire-Resistant Fluid Compatibility Chart

*Compatibility between products in the same fluid family must still be confirmed by the manufacturers.

7. Hydraulic Oil Compatibility with Seals and Elastomers

Hydraulic seals must retain their dimensions, strength, elasticity, and sealing force while continuously exposed to the system fluid. An incompatible oil can change the physical properties of an elastomer even when the fluid continues to perform normally as a lubricant.

Compatibility depends on:

• Fluid base stock
• Additive chemistry
• Operating temperature
• Exposure time
• System pressure
• Seal compound formulation
• Fluid contamination
• Dynamic or static seal service

The elastomer name alone is not always sufficient. For example, two NBR compounds can contain different acrylonitrile contents, fillers, plasticizers, and curing systems, resulting in different fluid resistance.

General Seal Compatibility Chart

These ratings provide general guidance only. The seal manufacturer should confirm the suitability of the exact compound and fluid combination.

Nitrile Rubber

Nitrile rubber, commonly identified as NBR or Buna-N, is one of the most widely used hydraulic sealing materials.

NBR generally provides good compatibility with:

• Mineral hydraulic oil
• HLP and HM anti-wear oils
• Diesel and petroleum-based fluids
• Many PAO synthetic fluids
• Some water-glycol fluids

Its resistance to synthetic esters, PAG fluids, and specialty fire-resistant fluids varies by compound. NBR is generally unsuitable for phosphate ester fluids because severe swelling, softening, or loss of mechanical strength may occur.

NBR also has a more limited high-temperature capability than HNBR or FKM.

Hydrogenated Nitrile Rubber

HNBR is a modified nitrile elastomer with better temperature, oxidation, ozone, and wear resistance than standard NBR.

It is commonly compatible with:

• Mineral hydraulic oils
• PAO fluids
• Many industrial lubricants
• Some biodegradable fluids
• Certain water-glycol formulations

Although HNBR performs well in demanding hydraulic applications, its compatibility with phosphate esters, PAG fluids, and some ester-based fluids must be verified.

Fluorocarbon Rubber

Fluorocarbon rubber, commonly known as FKM, offers excellent resistance to heat, mineral oil, fuels, PAO fluids, and many synthetic lubricants.

FKM is often selected for:

• High-temperature hydraulic systems
• Mineral and synthetic hydrocarbon oils
• Synthetic ester fluids
• Aggressive chemical environments

Standard FKM compounds may perform poorly in hot water, steam, and some water-glycol fluids. Specialized FKM grades may offer improved resistance, so the exact compound should be checked.

FKM is often suitable for phosphate ester fluids, but compatibility should not be assumed for every formulation.

EPDM Rubber

Ethylene propylene diene monomer, or EPDM, has excellent resistance to water, steam, glycol, weathering, ozone, and many phosphate ester fluids.

EPDM is commonly compatible with:

• Water-glycol hydraulic fluids
• High-water-content fluids
• Phosphate ester fluids
• Brake fluids based on glycol chemistry
• Hot water and steam

However, EPDM is generally incompatible with mineral oil, petroleum fluids, and PAO hydrocarbons. Contact with these fluids can cause severe swelling, softening, and loss of sealing strength.

EPDM seals should never be used in a conventional mineral-oil hydraulic system unless the seal manufacturer specifically approves the compound.

Polyurethane Seals

Polyurethane, commonly identified as PU or AU, provides excellent abrasion resistance, extrusion resistance, and mechanical strength. It is widely used for rod seals, piston seals, wipers, and high-pressure dynamic applications.

Polyurethane generally performs well with:

• Mineral hydraulic oils
• HLP and HM fluids
• PAO synthetic fluids

Its compatibility with water-glycol, ester, PAG, and phosphate ester fluids varies significantly. Some polyester-based polyurethane compounds are sensitive to hydrolysis in hot, wet environments. Polyether-based polyurethane may provide better water resistance.

The specific polyurethane chemistry must therefore be identified before use with water-containing fluids.

Silicone Rubber

Silicone rubber offers excellent flexibility at low temperatures and good resistance to heat. However, it has relatively low tear strength and may allow higher fluid permeation than other elastomers.

Its compatibility varies with hydraulic oil type and seal compound. Silicone is normally used in specialized static sealing applications rather than heavily loaded dynamic hydraulic seals.

PTFE Seals

Polytetrafluoroethylene, or PTFE, is highly resistant to nearly all common hydraulic fluid families. It is compatible with:

• Mineral oils
• PAO fluids
• Esters
• PAG fluids
• Water-glycol fluids
• Phosphate esters

PTFE has low friction and excellent chemical and temperature resistance. However, unmodified PTFE has limited elasticity and may experience cold flow or creep. PTFE hydraulic seals are therefore frequently energized by an elastomer O-ring or manufactured from filled PTFE compounds.

The energizing O-ring must be separately checked for fluid compatibility. A PTFE sealing ring may be compatible while the NBR or FKM energizer behind it is not.

Signs of Seal and Fluid Incompatibility

Common indicators include:

• Rapid seal swelling
• Shrinkage and loss of compression
• Soft, sticky, or gummy surfaces
• Hardening and cracking
• Blistering
• Loss of tensile strength
• Excessive extrusion
• Increased internal leakage
• Oil leaks around shafts, rods, or fittings
• Seal particles found in filters

Not every leak appearing after an oil change is caused by chemical attack. A new fluid may have a lower viscosity or stronger cleaning ability, exposing previously blocked leak paths or removing deposits from worn seals.

Seal Compatibility Testing

When published compatibility information is unavailable, sample seals can be immersed in the proposed fluid under controlled conditions. Testing may evaluate changes in:

• Volume
• Mass
• Hardness
• Tensile strength
• Elongation
• Surface condition
• Compression set

Testing should use the actual seal compound, not merely a generic elastomer of the same family. Exposure temperature and duration should represent the intended operating conditions.

As a practical rule, no hydraulic fluid conversion should proceed until the fluid has been checked against every wetted elastomer, including O-rings, shaft seals, accumulator bladders, diaphragms, hoses, reservoir gaskets, valve seals, and filter-element adhesives.

8. Can Different Hydraulic Oils Be Mixed?

Different hydraulic oils may be physically miscible, but that does not mean they are chemically compatible, interchangeable, or suitable for continued operation. In most cases, intentional mixing should be avoided unless the equipment manufacturer and both fluid suppliers confirm compatibility.

Oils that share the same viscosity grade can still differ in:

• Base-stock chemistry
• Anti-wear technology
• Oxidation inhibitors
• Detergent and dispersant additives
• Viscosity-index improvers
• Antifoam agents
• Corrosion inhibitors
• Water-separation characteristics
• Seal compatibility

For example, two products labeled ISO VG 46 may include a mineral HM oil, a high-viscosity-index HV fluid, a synthetic ester, or a water-glycol fire-resistant fluid. Their viscosity grades may be similar, but their chemistry and applications are completely different.

When Mixing May Be Possible

Limited mixing may be acceptable when both products:

• Use the same type of base stock
• Have the same viscosity grade
• Meet equivalent performance classifications
• Use compatible additive systems
• Are approved for the same equipment
• Have been confirmed as compatible by their manufacturers

Two conventional mineral-based HM or HLP oils are more likely to be compatible than fluids from different chemical families. However, even these oils may use different anti-wear technologies, such as zinc-containing and ashless additives.

A small top-up with an approved equivalent fluid may be possible when the original product is unavailable. Supplier confirmation should still be obtained, especially for critical or high-pressure equipment.

Oils That Should Not Be Mixed

The following combinations should generally be treated as incompatible unless specific testing proves otherwise:

• Mineral oil and water-glycol fluid
• Mineral oil and phosphate ester fluid
• Mineral oil and PAG fluid
• PAO and PAG fluid
• Ester fluid and water-glycol fluid
• Phosphate ester and conventional mineral or synthetic hydrocarbon oil
• Water-based fluids from different families
• Detergent hydraulic oil and conventional demulsifying hydraulic oil
• Zinc-containing and ashless fluids without compatibility confirmation
• Biodegradable fluids based on different base stocks

Mixing these fluids can cause phase separation, additive precipitation, sludge, foam, filter blockage, loss of fire resistance, seal damage, and reduced lubrication.

Can Different Hydraulic Oil Brands Be Mixed?

Brand names alone do not determine compatibility. Different brands may be compatible if their base stocks, viscosity grades, performance levels, and additive technologies are sufficiently similar.

However, products marketed under the same general description may still use different formulations. Mixing brands can also make future oil-analysis results more difficult to interpret because additive-element concentrations will change.

If an alternative brand must be used, compare:

• Product and safety data sheets
• ISO viscosity grade
• ISO and DIN classifications
• OEM approvals
• Base-oil type
• Zinc or ashless anti-wear chemistry
• Viscosity index
• Demulsibility
• Air-release and foam performance
• Seal compatibility

The most reliable confirmation should come from the oil suppliers rather than from appearance, color, or product name.

Can Different Viscosity Grades Be Mixed?

Oils with different viscosity grades may be miscible when they use compatible base stocks, but the blend will have an altered viscosity. The resulting value is not necessarily a simple arithmetic average.

Mixing ISO VG 32 with ISO VG 68 will produce an intermediate viscosity, but the blend may not reliably meet the limits or performance requirements of ISO VG 46.

An incorrect viscosity can cause:

• Difficult cold starts
• Pump cavitation
• Poor lubricating-film thickness
• Increased internal leakage
• Slow actuator response
• Reduced efficiency
• Excessive operating temperature

Different viscosity grades should only be mixed under controlled engineering guidance.

Can Synthetic and Mineral Hydraulic Oils Be Mixed?

Some PAO-based synthetic hydraulic oils are physically miscible with mineral oil. Nevertheless, mixing usually reduces the advantages of the synthetic fluid, including oxidation resistance, low-temperature performance, and extended service life.

Synthetic ester, PAG, phosphate ester, and other specialty fluids should not be assumed compatible with mineral oil. The term synthetic covers several unrelated chemistries and cannot be used as the sole basis for a mixing decision.

Can New and Used Hydraulic Oil Be Mixed?

Adding new oil to used oil is normal during routine top-up, provided that the new oil is the same approved product and the existing fluid remains serviceable.

However, new oil will not restore severely degraded fluid. It may dilute—but cannot remove:

• Oxidation products
• Water
• Dirt and wear particles
• Depleted additives
• Acids
• Varnish precursors
• Microbial contamination

If the existing oil has failed its condition limits, the system should be drained, cleaned where necessary, refilled, filtered, and investigated for the source of contamination.

What to Do After Accidental Mixing

If incompatible or unidentified oil has been added:

1. Stop adding the fluid immediately.
2. Record the product, amount, and point of entry.
3. Estimate the concentration in the total system volume.
4. Avoid operating the system if serious incompatibility is possible.
5. Contact the equipment and fluid suppliers.
6. Take a representative oil sample.
7. Test for viscosity, water, particle count, acid number, and physical instability.
8. Inspect for cloudiness, sediment, foam, or phase separation.
9. Drain and flush the system if compatibility cannot be confirmed.
10. Replace affected filters and inspect sensitive components.

The equipment should not continue operating simply because the mixture still appears clear. Some compatibility failures only become visible after heating, cooling, exposure to water, or extended service.

9. How to Change to a Different Hydraulic Oil Safely

Changing hydraulic oil safely involves more than draining the reservoir and adding a new product. A significant amount of old oil can remain inside cylinders, motors, pumps, accumulators, hoses, filters, coolers, and piping.

A poorly managed conversion can contaminate the new fluid, damage seals, block filters, and compromise equipment reliability.

Step 1: Define the Reason for the Change

Identify why the existing fluid is being replaced. Common reasons include:

• Product discontinuation
• Change in viscosity requirements
• Improved low- or high-temperature performance
• Conversion to a biodegradable fluid
• Need for fire resistance
• Extended drain intervals
• OEM recommendation
• Correcting use of the wrong oil
• Excessive varnish or deposit formation

The reason determines whether a simple same-product oil change or a complete chemical conversion is required.

Step 2: Identify the Existing Fluid

Collect as much information as possible about the fluid currently in the system:

• Manufacturer and product name
• Base-stock type
• ISO viscosity grade
• ISO or DIN classification
• Additive technology
• Length of service
• Oil-analysis history
• Top-up oils previously added
• Contamination condition

If the fluid is unidentified, laboratory analysis may be needed. Changing fluids without knowing the existing chemistry creates unnecessary risk.

Step 3: Select the New Fluid

Confirm that the proposed fluid meets:

• Pump and component requirements
• OEM specifications and approvals
• Required viscosity grade
• Operating-temperature range
• Minimum anti-wear performance
• Seal and hose compatibility
• Filtration requirements
• Fire-resistance requirements
• Environmental regulations

A biodegradable or fire-resistant fluid should also have the appropriate classification for the application. Marketing descriptions alone are insufficient.

Step 4: Obtain Compatibility Approval

Consult the equipment manufacturer, new-fluid supplier, and existing-fluid supplier where possible. Request written guidance on:

• Fluid-to-fluid compatibility
• Maximum allowable residual concentration
• Flushing-fluid selection
• Seal and hose suitability
• Paint and coating compatibility
• Filter compatibility
• Recommended commissioning tests

For critical equipment, conduct laboratory compatibility testing using representative mixing ratios.

Step 5: Establish a Conversion Plan

Prepare a written procedure covering:

• System shutdown and isolation
• Oil-drain locations
• Cylinder and accumulator draining
• Required flushing volume
• Filter replacement
• Seal replacement
• Reservoir cleaning
• Waste-fluid handling
• Sampling locations
• Acceptance criteria
• Restart and monitoring

The plan should also identify the person responsible for each activity and prevent accidental refilling with the old product.

Step 6: Take a Baseline Oil Sample

Before draining the system, take a representative sample from a live or turbulent zone. Avoid sampling stagnant oil from the bottom of the reservoir unless investigating settled contamination.

Typical baseline tests include:

• Kinematic viscosity
• Water content
• ISO cleanliness code
• Acid number
• Elemental analysis
• Oxidation indicators
• Membrane patch or varnish potential
• Visual appearance

The results provide a reference for evaluating the new fluid after conversion.

Step 7: Drain the Existing Oil While Warm

Operate the system until the oil reaches a safe operating temperature. Warm oil drains more completely and carries more suspended contamination out of the system.

Then:

1. Shut down and isolate the equipment.
2. Relieve all stored hydraulic pressure.
3. Follow lockout and tagout procedures.
4. Drain the reservoir.
5. Open low-point drains where provided.
6. Drain coolers, filter housings, manifolds, and large piping.
7. Retract or extend cylinders as required to expel trapped fluid.
8. Safely discharge and isolate accumulators according to manufacturer instructions.

Hydraulic accumulators and elevated loads can retain dangerous stored energy even after the pump has stopped.

Step 8: Clean the Reservoir and Accessible Components

Remove sludge, varnish, water, and settled particles from the reservoir. Use lint-free materials and cleaning products approved for the new fluid.

Inspect:

• Reservoir interior
• Suction strainers
• Magnets
• Breathers
• Access-cover gaskets
• Return diffusers
• Cooler surfaces
• Drain plugs
• Visible hoses and seals

Do not leave solvent or cleaning-fluid residue inside the system unless it is confirmed compatible with the new oil.

Step 9: Replace Filters and Incompatible Materials

Install new filter elements before flushing. If the fluid chemistry changes substantially, confirm compatibility of:

• Filter media
• Adhesives
• End caps
• O-rings
• Hoses
• Accumulator bladders
• Diaphragms
• Shaft seals
• Reservoir coatings
• Thread sealants

Seal replacement is particularly important when converting between mineral oil, water-glycol, and phosphate ester fluids.

Step 10: Flush the System

Select a flushing fluid approved by the new-fluid supplier. For conversions between compatible mineral oils, the new oil may be used for flushing. More difficult conversions may require an intermediate flushing product.

A typical process is:

1. Add sufficient flushing fluid to operate the system safely.
2. Circulate it through all lines and components.
3. Operate cylinders and motors through their full range.
4. Open individual circuit branches where possible.
5. Monitor filter differential pressure.
6. Replace blocked filters when necessary.
7. Drain the flushing fluid while warm.
8. Repeat until the residual old-fluid concentration meets the target.

Operating pressure and load should remain controlled during flushing. The purpose is to circulate fluid and remove contamination, not to place the machine into full production.

Step 11: Fill with Filtered New Oil

New hydraulic oil is not necessarily clean enough for immediate use in a precision system. Transfer the oil through a suitable filter using a dedicated or thoroughly cleaned transfer cart.

During filling:

• Use sealed and correctly labeled containers
• Clean hoses and couplings
• Filter the oil to the required cleanliness level
• Prevent water and airborne dirt from entering
• Avoid using shared, contaminated funnels or pumps
• Fill to the correct reservoir level

Step 12: Bleed Air and Restart Gradually

Air may remain trapped in pumps, actuators, and high points after draining. Follow the component manufacturer’s priming and venting instructions.

Start the system at low pressure and no load where possible. Then:

• Check pump noise
• Verify oil level
• Inspect for leaks
• Monitor filter differential pressure
• Observe foam and air release
• Cycle actuators slowly
• Check operating temperature
• Increase load progressively

Stop the system if there is abnormal noise, excessive foam, unstable movement, rapid filter blockage, or leakage.

Step 13: Sample and Monitor the New Fluid

Take follow-up samples after the fluid has circulated sufficiently. Additional sampling may be appropriate after the first several operating hours and again after the initial service period.

Monitor:

• Viscosity
• Water content
• Particle cleanliness
• Acid number
• Additive elements
• Residual old-fluid markers
• Foam and air release
• Filter condition
• Seal leakage
• Operating temperature

When converting to a fluid with stronger detergency or solvency, expect filters to collect loosened deposits. Shorter initial filter-change intervals may be necessary.

Step 14: Update Labels and Maintenance Records

Clearly label the reservoir, fill points, storage containers, transfer equipment, and maintenance documentation with the new product.

Record:

• Conversion date
• Previous and new fluid
• Quantity filled
• Flushing method
• Filters and seals replaced
• Laboratory results
• Residual-fluid concentration
• Supplier approvals
• Future sampling schedule

Remove or quarantine the previous oil from the maintenance area to prevent accidental top-up.

Conclusion

Hydraulic oil compatibility involves more than matching viscosity grades or selecting products with similar descriptions. The base stock, additive chemistry, performance classification, seal material, operating temperature, and equipment requirements must all be evaluated.

Mineral HM and HLP oils are often more likely to be mutually compatible, but mixing can still weaken anti-wear protection, water separation, air release, or viscosity performance. PAO fluids may be miscible with mineral oil, while PAG, water-glycol, and phosphate ester fluids generally require strict separation and a carefully controlled conversion procedure. Biodegradable and HFDU fluids must be identified by their actual base chemistry rather than by their general category alone.

The safest practice is to use one approved hydraulic fluid and prevent cross-contamination through clear labeling, dedicated transfer equipment, and controlled maintenance procedures. When changing oil types, obtain supplier and OEM approval, verify seal compatibility, drain and clean the system, flush trapped fluid, replace filters, refill with filtered oil, and monitor the system closely after startup.

A compatibility chart is useful for preliminary screening, but it cannot account for every commercial formulation. If compatibility is uncertain, laboratory testing and a complete system flush are far less costly than pump failure, blocked valves, damaged seals, or an unplanned production shutdown.

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