Can You Mix Hydraulic Oils? When It Is Safe and When It Is Not
Only if both hydraulic oils share base fluid, classification and viscosity grade, and the supplier confirms in writing that they are compatible. Never mix mineral oil with polyglycol (HEPG) or water glycol (HFC) fluids, or zinc with zinc-free oils. Without written confirmation, use a lab compatibility test or drain and flush instead.
Bosch Rexroth treats a hydraulic fluid that contains more than 2% of another fluid as a mixture (Bosch Rexroth RE 90220, 2012).
ASTM D7155 does not evaluate wear prevention, load-carrying capacity or shear stability, so a pass does not clear an antiwear hydraulic oil mix (ASTM D7155-20(2026)).
ASTM D7155 checks a mix in 4 test tiers, and passing Tier 1 alone shows only visual compatibility (ASTM D7155-20(2026)).
A changeover to HFC water glycol flushes with zinc-free oil at 32 °C to 38 °C for 1.5 to 2 hours, then skims oil from the reservoir for 7 to 10 days (Quaker Houghton, 2021).
When is it safe to mix hydraulic oils, and when not?
Mixing hydraulic oils is safe only when the two oils match in base fluid, classification and viscosity grade and the oil supplier confirms compatibility; every other combination is a no or needs a planned conversion. Compatibility means more than miscibility: two oils can blend into one clear liquid while their additive packages still react, so a data sheet that says "miscible" does not settle the question (Hansa-Flex, 2025). The table below decides each common case for industrial hydraulic systems; mobile and vehicle systems follow their own equipment manual.
Condition | Verdict | Why |
|---|---|---|
Same classification and grade, other brand (for example two mineral HM oils) | Only if the supplier confirms compatibility in writing or a lab test passes | Additive packages differ; more than 2% of another fluid counts as a mixture (Bosch Rexroth RE 90220, 2012) |
Same product line, different ISO viscosity grade (antiwear AW 32 into AW 46) | No; top up with the same grade | The blend lands between grades (ASTM D7152-23) and may miss the pump's viscosity limits; Hansa-Flex lists it as never mix (Hansa-Flex, 2025) |
Zinc with zinc-free oil | No | The two additive systems are most often incompatible, even at low ratios (Bosch Rexroth RE 90220, 2012) |
Detergent (HLPD) with non-detergent (HLP) oil | No | Listed as never mix (Hansa-Flex, 2025) |
Mineral oil with HEES synthetic ester | No as a top-up; only as a flushed conversion | Residual mineral oil under 2% (Bosch Rexroth RE 90221, 2002) |
Mineral oil with HEPG polyglycol | No | Incompatible; residual mineral oil under 1% after conversion (Bosch Rexroth RE 90221, 2002) |
Two biodegradable fluids of different brands or types (for example HEES with HEES, or HEES with HETG) | No | Listed as never mix (Hansa-Flex, 2025); Rexroth says to avoid mixing even different types of ester fluids (Bosch Rexroth RE 90221, 2002) |
Mineral oil with HFC water glycol | No | Requires full drain, flush and skimming of oil (Quaker Houghton, 2021) |
Equipment maker forbids mixing | Follow the maker | Rexroth accepts no liability for damage from mixing |
The key row is the first one: two mineral oils of the same classification and grade from different brands mix safely only once the oil supplier confirms compatibility in writing or a lab test passes. Every verdict also assumes the oil already in the system still meets its viscosity, acid and oxidation limits (Hansa-Flex, 2025); new oil added to degraded or contaminated in-service oil may not be compatible even when the two new oils are (ASTM D7155-20(2026)).
Why do sources disagree on mixing hydraulic oil brands?
Sources disagree on mixing hydraulic oil brands because they answer different questions: whether two oils dissolve into each other, or whether the mix still performs in a specific machine. Hansa-Flex advises avoiding any mix, since no combination is guaranteed to be problem-free, and names oils of one classification but different viscosities as a mix to avoid (Hansa-Flex, 2025). Its conclusion still allows a mix, except with PAG-based fluids, once the oil supplier confirms compatibility, preferably in writing, or a lab analysis does. Bosch Rexroth does not generally permit mixing, even within the same classification, and leaves any miscibility claim to the lubricant maker (Bosch Rexroth RE 90220, 2012). ExxonMobil's data sheet shows where chemistry alone draws the line: its Mobil Glygoyle polyalkylene glycol (PAG) gear and compressor oils are not compatible with mineral oils and most other synthetic lubricants, and it generally prefers draining, flushing and refilling to mixing, even between PAG oils (ExxonMobil, 2025). ASTM D7155 sits between Rexroth's no and Hansa-Flex's conditional yes, treating a mix as acceptable only after a tiered lab evaluation. That practice is written for turbine oils, so for hydraulic oils its tiers serve as a borrowed lab check, not a hydraulic standard (ASTM D7155-20(2026)). The likely reason for the split is liability: a component maker carries the warranty risk, while an oil supplier judges chemistry.
What happens if you mix incompatible hydraulic oils?
Mixing incompatible hydraulic oils most often forms a precipitate that deposits in the system, plugs filters and blocks oil passages (ASTM D7155-20(2026)). Bosch Rexroth reports gelling, silting and deposits after mixing oils from different makers or different types from one maker, which then cause foaming, poor air separation, malfunctions and damage (Bosch Rexroth RE 90220, 2012). Hansa-Flex adds surface foaming, slower air release, seal and hose wear, and clogging from zinc soap (Hansa-Flex, 2025). A mix can also lose demulsibility, foam control, oxidation stability, rust protection or antiwear protection even when it passes a visual check (ASTM D7155-20(2026)). Damage can arrive late: Hansa-Flex warns that faults from incompatible hydraulic oils may appear only indirectly or in the long term, so a mix that ran well for a week is not proof of compatibility. If an unplanned mix has happened, take an oil sample before deciding whether to keep it.
What should you do instead of mixing hydraulic oils?
Instead of mixing hydraulic oils, top up only with the exact product already in the system, and treat any change of product as a planned changeover. When the original oil is unavailable, ask the new supplier for a written compatibility statement; Rexroth recommends a written performance guarantee from the new oil supplier (Bosch Rexroth RE 90220, 2012). Without that statement, order a lab compatibility test that compares the mix with each neat oil, including air release, water separation, foaming, acid content and filterability at several mixing ratios (Hansa-Flex, 2025; ASTM D7155-20(2026)). For a move to HEES, HEPG or water glycol, drain and flush to the residual limits in the table rather than topping up. Drain an unplanned mix into its own labeled container and dispose of it per the safety data sheet (SDS) and the local used-oil rule, such as US 40 CFR 279.10(b)(1)(ii), under which used oil above 1,000 ppm total halogens is presumed hazardous waste unless the presumption is rebutted (eCFR, 2026). Call a lubrication specialist when the machine is under warranty or the mix already went in. Lubinpla can work that decision from your own technical data sheets, SDS and site history and return one cited document for your review.
FAQ
Can you mix hydraulic fluid brands?
Only if both oils share base fluid, classification and viscosity grade and the new supplier confirms compatibility in writing. Bosch Rexroth does not generally permit mixing even within one classification (Bosch Rexroth RE 90220, 2012).
Can you mix HLP and HVLP hydraulic oil?
No. Hansa-Flex lists hydraulic oils with different classifications, such as HLP, HLPD and HVLP, as never mix (Hansa-Flex, 2025). Treat a change of classification as a planned changeover with the new supplier's written approval.
Can you mix AW 32 and AW 46 hydraulic oil?
No. Top up with the grade already in the system. After an accidental mix, ASTM D7152-23 can estimate the blend viscosity of oils without viscosity index improvers; for HVLP oils, measure it. Then check the result against the pump maker's limits.
Working a case like this? Lubinpla works it from your own documents and site history, step by step with your approval. Free to start, no card.
Sources checked 2026-10-13.
Sources
Hansa-Flex. No two hydraulic oils are the same! Types of hydraulic oil and whether they mix. 2025. https://www.hansa-flex.de/en/magazine/details/no-two-hydraulic-oils-are-the-same-types-of-hydraulic-oil-and-whether-they-mix/
Bosch Rexroth. RE 90220/05.12 Hydraulic fluids based on mineral oils and related hydrocarbons. 2012. https://hydraulique-martin.com/wp-content/uploads/2021/02/Bosch-Hydraulic-Fluid-Selection.pdf
Bosch Rexroth. RE 90221/01.02 Environmentally acceptable hydraulic fluids HEES, HEPG, HETG for axial piston units. 2002. https://www.etshydro.com/images/stories/TechDataSheets/data%20enviromentally%20acceptable%20oil%20heeshepghetg%20for%20axial%20units%20re90221.pdf
ASTM International. ASTM D7155-20(2026) Standard Practice for Evaluating Compatibility of Mixtures of Turbine Lubricating Oils. 2026. https://store.astm.org/d7155-20r26.html
ASTM International. ASTM D7152-23 Standard Practice for Calculating Viscosity of a Blend of Petroleum Products. 2023. https://store.astm.org/d7152-23.html
Quaker Houghton. Recommended conversion procedure: water free hydraulic fluid to HFC water glycol. 2021. https://store.quakerhoughton.com/media/pdfs/assembly/skb_water-free-hydraulic-fluid_to_HFC_conversion_procedure_EN.pdf
ExxonMobil. Mobil Glygoyle Series product data sheet, Compatibility with other lubricants. 03-2025. https://www.mobil.com/en-vn/industrial/pds/gl-xx-mobil-glygoyle-series
US Government Publishing Office, eCFR. 40 CFR Part 279 Standards for the Management of Used Oil. Current as of 2026-10-08. https://www.ecfr.gov/current/title-40/chapter-I/subchapter-I/part-279
Written by the Lubinpla team from the sources listed and checked against them before publication.