Types of Hydraulic Oil and Where Each One Should Be Used

Choose HM antiwear oil for most industrial systems, HV for wide temperature swings, HFx fluids near ignition sources, and HEx fluids where leaks reach water. These codes come from the ISO 6743-4 classification. The pump maker's approved fluid list and the fluid's specification standard, ISO 11158, ISO 12922 or ISO 15380, settle the final choice.

• ISO 11158:2023 covers five mineral oil hydraulic categories, HH, HL, HM, HV and HG, as minimum requirements at the time of delivery (ISO 11158:2023).

• ASTM D6158-23 defines Class HV as an antiwear hydraulic oil with a viscosity index higher than 140 (ASTM D6158-23).

• Danfoss lists HFC water glycol fluids at more than 35 % water and runs its axial piston pumps on HFC at 60 % of rated differential pressure and 65 % of rated speed (Danfoss 520L0463, 2016).

• Ready biodegradability, one of the three criteria for environmentally acceptable lubricants, means 60 % of the material converted to CO2 within a 10-day window that falls inside 28 days of test start (US EPA, 2011).

This guide covers hydrostatic hydraulic systems in plants and mobile machines classified under ISO 6743-4:2015. It does not cover aircraft hydraulic fluids or automotive brake fluids, which ISO 6743-4 excludes, or power-generation control fluids, which ISO 12922:2020 excludes. Where a pump or valve data sheet names approved fluids, that list overrides the rules below.

Which conditions decide the hydraulic oil type?

Four conditions decide the hydraulic oil type: the wear protection the pump needs, the temperature range the oil must cover, the fire risk around the system, and where a leak would end up. Pump type comes first. Vane pumps "can suffer high rates of wear between the vanes and the cam ring if a suitable antiwear oil is not used" (STLE, 2016). Piston pumps reach higher pressures, and gear pumps tolerate more contamination but give relatively low pressures (STLE, 2016).

Temperature range comes second. Conventional hydraulic fluids have a viscosity index (VI) of around 100, while a multigrade hydraulic fluid should have a VI of at least 140 for outdoor machines that start cold and run hot (STLE, 2016).

Fire risk and leak path come last, and they override the first two. Near an ignition source, Danfoss recommends a fire-resistant fluid whenever there are fire hazards (Danfoss 520L0463, 2016). Where leaked oil reaches water or soil, an environmentally acceptable fluid limits the damage (US EPA, 2011). Both choices change the base fluid, so they also change seals, paints, filters and pump ratings. Rexroth asks users to check seal, hose, coating, metal and plastic compatibility for parts it did not supply (Bosch Rexroth RE 90220, 2012).

What do the ISO hydraulic fluid codes HL, HM and HV mean?

The ISO hydraulic fluid codes HL, HM and HV name three mineral oil performance levels in the ISO 6743-4 classification, each adding one property to the level before it. The first letter H marks family H, hydraulic systems, in ISO 6743-4:2015. ASTM D6158-23 spells out the levels in words:

• HL: refined base oil "with rust and oxidation inhibitors" (ASTM D6158-23).

• HM: HL performance "plus anti-wear characteristics" (ASTM D6158-23).

• HV: HM performance with a raised viscosity index; ASTM D6158-23 sets it higher than 140 (ASTM D6158-23).

ISO 11158:2023 sets the minimum requirements for these categories, plus HH and HG, and states the requirements at the time of delivery (ISO 11158:2023). ISO 6743-4 itself only names the categories. Bosch Rexroth notes that ISO 6743-4 "contains no information on minimum requirements for hydraulic fluids" (Bosch Rexroth RE 90220, 2012).

In German and many European data sheets, the same levels appear as HL, HLP and HVLP under DIN 51524 Parts 1, 2 and 3; Parts 2 and 3 are current in their 2017-06 editions (DIN 51524-2:2017; DIN 51524-3:2017). DIN 51524 is a separate set of minimum requirements, not a relabel of ISO 11158. Rexroth accepts ISO 11158 HL, HM or HV fluids without confirmation only when the component data sheet lists them and the fluid data sheet shows DIN 51524 conformity (Bosch Rexroth RE 90220, 2012).

Which hydraulic oil type do most industrial systems use?

Most industrial hydraulic systems use an HM antiwear mineral oil, called HLP under DIN 51524-2, in ISO VG 32, 46 or 68. "Hydraulic equipment used indoors under normal service conditions generally use a conventional antiwear hydraulic oil with the appropriate viscosity" (STLE, 2016). The same source names ISO 32, 46 and 68 as the most common viscosities (STLE, 2016).

Pump data sheets point the same way. Danfoss lists DIN 51524-2 HLP, DIN 51524-3 HVLP and ISO 11158 HM and HV as the mineral oil fluids for its hydrostatic units (Danfoss 520L0463, 2016). DIN 51524-2 applies where oils with antiwear additives are needed in pumps or hydraulic motors because of their design or operating conditions (DIN 51524-2:2017).

HL oil without antiwear additives has a narrow place. Rexroth states that HL fluids "can be used in hydraulic systems that do not pose any requirements as to wear protection", and only for components whose product data sheet specifically allows HL fluids (Bosch Rexroth RE 90220, 2012).

HV oil is the second common choice. Rexroth uses HVLP fluids in systems operated over a wide temperature range and recommends them only where the temperature range requires it (Bosch Rexroth RE 90220, 2012). The reason is shear: the VI improvers in HV oil break down in service, and Rexroth recommends a maximum viscosity drop of 15 % at 100 °C for fluids up to VI 160 (Bosch Rexroth RE 90220, 2012).

When do you need a fire-resistant hydraulic fluid?

You need a fire-resistant hydraulic fluid when a leak or hose burst could spray fluid onto an ignition source such as molten metal, a furnace, a hot surface or mining equipment underground. STLE names underground coal mines, steel works and glass making as typical cases, with "water-based fluids for low pressures and temperatures or phosphate esters for higher pressures and temperatures" (STLE, 2016).

ISO 12922:2020 sets the minimum requirements for the fire-resistant categories HFAE, HFAS, HFB, HFC, HFDR and HFDU. It treats HFAE, HFAS, HFB, HFC and HFDR as fire-resistant under ISO 5598, and says most HFDU fluids are more appropriately considered "less flammable" fluids (ISO 12922:2020). HSE fire tests for underground work point the same way: "Once a fluid is burning, the experimental data suggest that HFDU fluids give little, if any benefit over mineral oils, while HFB/HFC fluids give a significant advantage" (HSE RR1206, 2024).

The trade-off is hardware rating. Danfoss runs its axial piston pumps on HFC at 60 % of rated differential pressure and 65 % of rated speed, and on HFD at 100 % (Danfoss 520L0463, 2016). Danfoss rates fire-resistant fluids as unsuitable for its gear pumps and motors (Danfoss 520L0463, 2016). HFC fluids attack zinc and cadmium (Danfoss 520L0463, 2016). Read the safety data sheet (SDS) of the fluid and the local fire code before the change.

When do you need an environmentally acceptable hydraulic fluid?

You need an environmentally acceptable hydraulic fluid when leaked or spilled fluid can reach water or soil: vessels, forestry, agriculture and machines working near watercourses. US EPA defines environmentally acceptable lubricants (EALs) as lubricants "demonstrated to meet standards for biodegradability, toxicity and bioaccumulation potential" (US EPA, 2011). STLE suggests vegetable oil based fluids for low pressures and temperatures and synthetic esters for higher pressures and temperatures (STLE, 2016).

ISO 15380:2023 specifies four environmentally acceptable categories: HETG (triglycerides), HEPG (polyglycols), HEES (synthetic esters) and HEPR (polyalphaolefins and other synthetic hydrocarbons) (ISO 15380:2023). A biodegradable label alone is not the test: ask which standard (ISO 15380:2023) and which ecolabel the fluid meets, and whether it meets the three EPA criteria of biodegradability, toxicity and bioaccumulation potential (US EPA, 2011).

Each category has limits. Danfoss accepts HEES, HETG and HEPG fluids in its hydrostatic units only when they also meet Annex B of ISO 15380, a shear stability and yellow metal test (Danfoss 520L0463, 2016). Danfoss notes that HETG has limited temperature capability and recommends fluid changes every 1000 operating hours or once a year for HETG, against 2000 hours for mineral oil (Danfoss 520L0463, 2016). Plan the conversion from mineral oil with the changeover guidance that Rexroth points to: VDMA 24314, VDMA 24569 and ISO 15380 appendix A (Bosch Rexroth RE 90220, 2012).

Hydraulic oil type decision table

The hydraulic oil type decision table below maps each ISO 6743-4 category used in practice to its base fluid, the condition that calls for it, and the standard that sets its minimum requirements. It reads top to bottom: start from the HM row and move down only when a fire, water or temperature condition applies.

The row most plants need is HM: a mineral oil with rust and oxidation inhibitors plus antiwear additives, specified by ISO 11158:2023 or DIN 51524-2:2017 as HLP. Move to HV only for a real temperature swing, to an HF category only for an ignition risk, and to an HE category only for a leak path to water or soil. Within the fire-resistant rows, HFC trades pump rating for fire resistance, while HFD fluids keep the full rating on Danfoss piston pumps but require Viton (fluorocarbon) seals (Danfoss 520L0463, 2016).

Common mistakes when choosing a hydraulic oil type

The most common mistake when choosing a hydraulic oil type is reading the viscosity grade as the type: HL 46, HM 46 and HV 46 are three different oils in the same grade.

The second mistake is treating HFDU as fire-resistant. ISO 12922:2020 places most HFDU fluids outside the ISO 5598 definition of fire-resistant (ISO 12922:2020), and HSE found little benefit over mineral oil once the fluid is burning (HSE RR1206, 2024).

The third mistake is mixing additive systems. Zinc-containing and zinc-free hydraulic oils are "most often incompatible with each other", and Rexroth treats a fluid with more than 2 % of another fluid as a mixture (Bosch Rexroth RE 90220, 2012). Mixing is not generally permitted, even between fluids with the same classification (Bosch Rexroth RE 90220, 2012).

The fourth mistake is assuming food-grade means environmentally acceptable. Rexroth warns that fluids for the pharmaceutical and food industries "should not be confused with environmentally acceptable fluids" (Bosch Rexroth RE 90220, 2012).

The fifth mistake is choosing HV oil by habit. Its VI falls as the oil shears in service, and the pump's viscosity limits still apply after shear (Bosch Rexroth RE 90220, 2012).

When should you call a specialist about hydraulic oil type?

Call a hydraulics or lubrication specialist about hydraulic oil type before any change of base fluid, meaning a move from mineral oil to an HF or HE category, and whenever a pump data sheet does not list the fluid you plan to use. Rexroth tests its components with HLP oil before delivery and will not accept liability for damage from inadequate fluid changeovers or from mixing fluids (Bosch Rexroth RE 90220, 2012).

Bring the specialist four documents: the data sheet of every pump and motor in the circuit, the technical data sheet of the current fluid, its SDS, and a recent oil analysis. Ask the supplier which categories and editions the new fluid meets, for example ISO 11158:2023 HM and DIN 51524-2:2017 HLP, and whether the pump maker has approved it. STLE advises asking whether the fluid is "approved or recommended by the equipment manufacturer" (STLE, 2016).

A fire-resistant or environmentally acceptable conversion also touches seals, hoses, reservoir coatings and filters, and the plant's fire or environmental permit. Matching pump data sheets, fluid data sheets and SDSs into one decision record is the kind of case Lubinpla structures into one cited document for your review.

FAQ

Can you use engine oil or automatic transmission fluid as hydraulic oil?

Only where the pump maker allows it. Danfoss accepts some engine oils and automatic transmission fluids in its hydrostatic units, but each must also meet the requirements of Table 3 in ISO 11158 (Danfoss 520L0463, 2016). Rexroth allows automatic transmission fluids only after confirmation (Bosch Rexroth RE 90220, 2012).

Can you use turbine oil in a hydraulic system?

Sometimes, after the pump maker confirms it. Rexroth allows turbine oils after confirmation and notes they "usually offer lower wear protection than mineral oil HLP" (Bosch Rexroth RE 90220, 2012). Rexroth compares turbine oils to DIN 51515-1 with HL and those to DIN 51515-2 with HLP (Bosch Rexroth RE 90220, 2012).

Do fire-resistant and biodegradable hydraulic fluids need shorter change intervals?

Danfoss recommends shorter intervals for HFA, HFB, HFC, HFD and HETG fluids: after the first change at 500 hours, every 1000 operating hours or once a year, against 2000 hours for other fluids (Danfoss 520L0463, 2016). Oil analysis can justify a different interval.

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.

Lubinpla does not sell chemical products and names no brands in recommendations. Our reviewer also leads a specialty chemicals distribution company.

Sources checked 2026-10-13.

Sources

1. ISO. ISO 6743-4:2015 Lubricants, industrial oils and related products (class L): Classification: Part 4: Family H (Hydraulic systems). Edition 3, 2015, confirmed 2026. https://committee.iso.org/standard/60483.html

2. ISO. ISO 11158:2023 Lubricants, industrial oils and related products (class L): Family H (hydraulic systems): Specifications for categories HH, HL, HM, HV and HG. Edition 3, 2023 (SIS catalog page). https://sis.se/en/produkter/petroleum-and-related-technologies/hydraulic-fluids/iso-111582023

3. ISO. ISO 12922:2020 Lubricants, industrial oils and related products (class L): Family H (hydraulic systems): Specifications for hydraulic fluids in categories HFAE, HFAS, HFB, HFC, HFDR and HFDU. Edition 3, 2020, confirmed 2025. https://committee.iso.org/standard/76807.html

4. ISO. ISO 15380:2023 Lubricants, industrial oils and related products (class L): Family H (Hydraulic systems): Specifications for hydraulic fluids in categories HETG, HEPG, HEES and HEPR. Edition 4, 2023. https://committee.iso.org/standard/82967.html

5. ASTM International. ASTM D6158-23 Standard Specification for Mineral Hydraulic Oils. 2023. https://store.astm.org/d6158-23.html

6. DIN. DIN 51524-2:2017-06 Pressure fluids: Hydraulic oils: Part 2: HLP hydraulic oils, minimum requirements. 2017. https://www.dinmedia.de/en/standard/din-51524-2/272262662

7. Bosch Rexroth. RE 90220/05.12, Hydraulic fluids based on mineral oils and related hydrocarbons. 2012 (edition read). https://hydraulique-martin.com/wp-content/uploads/2021/02/Bosch-Hydraulic-Fluid-Selection.pdf

8. DIN. DIN 51524-3:2017-06 Pressure fluids: Hydraulic oils: Part 3: HVLP hydraulic oils, minimum requirements. 2017. https://www.dinmedia.de/en/standard/din-51524-3/272262707

9. Danfoss. 520L0463, Hydraulic Fluids and Lubricants, Technical Information. July 2016. https://assets.danfoss.com/documents/latest/54030/BC152886484524en-000902.pdf

10. STLE (R. David Whitby). Selecting the correct hydraulic oil. Tribology and Lubrication Technology, January 2016. https://www.stle.org/images/pdf/STLE_ORG/BOK/LS/Hydraulics/Selecting%20the%20Correct%20Hydraulic%20Oil_Jan16%20tlt.pdf

11. Health and Safety Executive (HSE). RR1206 Fire safety: hydraulic systems used in underground work. 2024. https://www.hse.gov.uk/research/assets/docs/rr1206.pdf

12. US Environmental Protection Agency. Environmentally Acceptable Lubricants, EPA 800-R-11-002. November 2011. https://www.epa.gov/sites/default/files/2020-02/documents/vgp_environmentally_acceptable_lubricants1.pdf

Written by the Lubinpla team from the sources listed and checked against them before publication.