Short answer
ISO 6743-4 is the ISO classification of hydraulic fluids, the Family H of industrial lubricants. Its letter codes describe fluid types: HM is an anti-wear mineral oil and HV is an anti-wear oil with improved viscosity-temperature behavior. The detailed test limits for the mineral oil categories are set in a companion standard, ISO 11158.
A classification, not a test list
ISO 6743-4 belongs to the ISO 6743 series, which sorts industrial lubricants (class L) into families by application. Family H covers hydraulic systems; the standard excludes automotive brake fluids and aircraft hydraulic fluids. Each fluid type gets a code whose first letter, H, identifies the family, as in ISO-L-HM 46, where 46 is the ISO VG grade.
The classification tells you what kind of fluid something is. The minimum performance requirements for the mineral oil types are published separately in ISO 11158, while fire-resistant fluids are covered by ISO 12922 and environmentally acceptable fluids by ISO 15380.
The mineral oil codes
The commonly used mineral oil categories are HH, a refined oil without inhibitors; HL, which adds rust and oxidation inhibitors; HM, which adds anti-wear performance on top of HL; HV, an HM-type oil with improved viscosity-temperature properties; and HG, an HM-type oil with anti-stick-slip properties for combined hydraulic and slideway systems.
HM is the workhorse category for industrial and mobile hydraulics with gear, vane or piston pumps. HV is used where one oil has to work across wide temperature swings, such as outdoor mobile equipment or machines started cold and run hot.
Fire-resistant and environmentally acceptable codes
The same family includes fire-resistant fluids such as HFAE, HFAS, HFB, HFC, HFDR and HFDU, used where a leak could reach an ignition source, and environmentally acceptable fluids: HETG (triglycerides, such as vegetable oils), HEPG (polyglycols), HEES (synthetic esters) and HEPR (polyalphaolefins and related hydrocarbons).
These are not drop-in substitutes for HM oils. Seal compatibility, water tolerance and conversion procedures differ, so switching to or from one of these types should follow the equipment maker’s instructions.
How ISO codes relate to DIN and AW labels
Many oils carry more than one label. In broad terms an ISO HM oil corresponds to the DIN 51524 HLP type and an ISO HV oil to HVLP, and in North America the same products are often sold as “AW” or “anti-wear” hydraulic oil. The categories are similar in intent, but each standard has its own test list and limits, so a manual that names a specific standard should be matched to that standard.
Frequently asked questions
Is HM the same as AW hydraulic oil?
“AW” is an informal market term for anti-wear hydraulic oil. ISO HM is a formal category with requirements in ISO 11158. Many AW oils are sold as meeting HM, but check the product data rather than assuming.
What does ISO-L-HV 46 mean?
Class L (lubricants), family H (hydraulic), type V (anti-wear with improved viscosity-temperature behavior), ISO viscosity grade 46.
General maintenance information. Always follow your vehicle or equipment manufacturer’s requirements and the current product data. Questions? Email [email protected].
