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Hydraulic Oil Classifications and Information

Hydraulic oils are needed to transmit energy in hydraulic systems. Discover hydraulic oil classifications and the different types of fluids.

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Hydraulic Oil Classifications

Hydraulic oil is a liquid that’s needed to transmit energy in hydraulic systems.

Requirements for hydraulic liquids – especially in construction machinery:

  • Good lubrication properties
  • High resistance to ageing
  • High wetting capacity and adhesive power
  • High flash point
  • Low pour point (lowest temperature at which oil is still liquid; e.g. -5oC)
  • Must not affect gaskets
  • Resin and acid-free
  • Low influence of temperature on viscosity – both dynamic viscosity (which usually increases as the temperature rises) and kinematic viscosity (the relationship between the dynamic viscosity and the density)
  • Low compressibility
Hydraulic Classification

For more information on hydraulic fluids, visit our Hydraulic Oil Guide. You may also find Crown Oil’s hydraulic oil product specs useful.

Hydraulic fluids are formulated according to the application and required properties.

Mineral oil-based

The most popular hydraulic liquid (hydraulic oil) is mineral oil-based, with suitable additives added as per the application. The requirements for this hydraulic oil are set out in ISO 6743-4 with the designations HL, HM, HV. The German designations HL, HLP, HVLP are used universally for hydraulic oils, in accordance with DIN 51524.

H and HHMineral oil with no active ingredients
HLContains active ingredients to enhance corrosion protection and resistance to ageing
HMIncludes active ingredients to improve corrosion protection and resistance to ageing as well as to reduce wear from scoring in the mixed friction area
HLPAdditional active ingredients as well as HL oil are added to reduce wear and increase resistance in the mixed friction area – the most common application in practice
HV and HVLPSimilar to HLP, but with increased resistance to ageing and an improved temperature-viscosity relationship
HLPDLike HLP, but with additives to enhance particle transport (detergent effect) and dispersion capacity (water carrying capacity) and active ingredients to increase the corrosion protection (German designation, not standardised)

Fire-resistant fluids

HFAE: Oil in water emulsionsThe water content is above 80% and is mixed with a mineral oil or soluble polyglycol-based concentrate
With a mineral-oil based concentrate, there is the risk of separation and microbe growth (diesel bug)
Flame resistant, can be used at temperatures between +5°C and +55°C
HFAS: Synthetic concentrates dissolved in waterNo risk of separation, as this is a true solution, which means the hydraulic components are significantly more susceptible to corrosion
HFB: Water in oil emulsionsThe water content is above 40% and is mixed with a mineral oil – this emulsion is rarely used
Flame resistant and can be used at temperatures between +5°C and +60°C
In Germany, HFB fluids are not permitted due to the lack of fire protection properties
HFC: Water glycolsThe water content is more than 35% in a polymer solution
Flame resistant and can be used at temperatures between -20°C and +60°C
Can be used at pressures of 250 bar
HFD: Synthetic liquidsHFD-R: Phosphoric esters
HFD-S: Anhydrous chlorinated hydrocarbons
HFD-T: Mixture of HFD-R and HFD-S
HFD-U: Anhydrous other composition (consisting of fatty acid esters)
Synthetic liquids have a higher density than mineral oil or water (not HFD-U), which can cause problems with the suction performance of pumps and affect a lot of gasket materials.
Flame resistant and can be used at temperatures between -20°C and +150°C


Biodegradable hydraulic liquids are manufactured using plant oils, such as rapeseed oil and are used in biologically critical environments, such as construction machinery in water protection areas and snow grooming equipment in mountains. The fluids are class 1 harmful substances.

Labelling: HE = Hydraulic Environmental


  • HETG (triglyceride base = plant oils)
  • HEES (synthetic ester base)
  • HEPG (polyglycol base)
  • HEPR (other base liquids, primarily Poly-alpha-olefins)


Water is inoffensive as a hydraulic liquid in every respect, however, it doesn’t offer corrosion protection. Pure water is not used in power hydraulics and is instead mixed with oil to form an emulsion, similar to cutting oil in cutting machines. However, there is the issue of separation in some instances.

The first technical use of hydraulics employed water as the fluid. Water has a virtually continuous low viscosity.


  • Tap water (filtered)
  • Technical water (water-oil emulsion)
  • Sea and saltwater (filtered, not suitable due to aggressiveness)

Discuss your hydraulic oil requirements with the Crown Oil’s team of lubricant specialists. Call 0330 123 1444 today.

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