April 15, 2026 BY
IN Oil

Hydraulic fluid requirements

Nowadays, hydraulic fluids face challenging requirements and standards through equipment manufacturers and customers applications. Several applications will need specific characteristic to meet the standard of the industry, OEM, and regulatory requirements.

Classification of hydraulic fluid

There are several classifications which defines hydraulic fluid for hydrostatic applications:

• ISO 6743/4 or DIN 51524/3 – applied to mineral based fluid, biodegradable fluid, and fire-resistant hydraulic fluid

• ISO 12922 – specifically applied to fire-resistant hydraulic fluid

• ISO 15380 – specification for environmentally friendly hydraulic fluids

• OEM requirements

Mineral oil based hydraulic fluids – ISO 6743/4

• ISO-L-HH: Mineral lubricants without corrosion inhibitors

• ISO-L-HL: HH lubricants with anti-rust and oxidation additive

• ISO-L-HM: HL lubricants with anti-wear additive

• ISO-L-HR: HL lubricants with a high VI ISO-L-HV HM lubricants with a high VI

• ISO-L-HG: HM lubricants with shock resistant features

• ISO-L-HS: Synthetic liquids

German OEMs usually also mention DIN 51524/3 standard which has slightly different specification from ISO 6743/4.

• DIN HL: Mineral oil with anti-rust and oxidation additive

• DIN HLP: HL oils with improved anti-wear properties

• DIN HVLP: HLP oils with improved temperature-viscosity behavior

• DIN HLPD: HLP oils with additives to improve detergency and dispersion capacity.

Environmentally friendly hydraulic fluids – ISO 15380

• DIN HETG: triglycerides (vegetable not soluble in water)

• DIN HEES: synthetic ester non-water soluble

• DIN HEPG: polyglycols types soluble in water

• DIN HEPR: polyalphaolefins and related hydrocarbon products.

Fire resistant hydraulic fluids – ISO 12922

• ISO HFAE: oil in water emulsions, mineral or synthetic ester (> 80% water content)

• ISO HFAS: mineral oil free, aqueous synthetic chemical solution (> 80% solution)

• ISO HFB: water in oil emulsions, mineral content >40%

• ISO HFC: water glycol solution, water content >35%

• ISO HFDR: water-free synthetic fluids consist of phosphate esters (not soluble in water)

• ISO HFDS: chlorinated hydrocarbons

• ISO HFDT: mixture HFDU / HFDT

• ISO HFDU: water-free synthetic fluids other than phosphate esters

OEM requirements

Several leading OEM components manufacturers such as Denison, Bosch and Vickers also issued their own standards usually related to thermal/oxidation stability, mechanical/ dynamic behavior, and filterability. They also have their own in-house tests.