An Introduction to Conventional Hydraulic Oils

The function of a hydraulic fluid is to provide a means of power transfer in a hydraulic circuit.  The fluid must lubricate the pumps and motors without impairing the function of the other components in the total hydraulic system. 

Basic Functions of Hydraulic Fluid

The basic functions of a hydraulic fluid are there to provide:

  • Rust Inhibition
  • Demulsibility
  • Filterability and compatibility
  • Hydrolytic stability
  • Corrosion Control
  • Oxidation Stability
  • Thermal Stability
  • Anti-wear

There are two classes of hydraulic Fluids:

  • Conventional Mineral
  • Fire Resistant 

Conventional Mineral Hydraulic Fluids:

Conventional mineral oils are further broken down as follows:


For hydraulic systems operating at low pressure, i.e. below 1,000psi, with maximum bulk fluid operating temperatures up to 82°C. Standard Conventional hydraulic fluids normally do not contain zinc antiwear additive technology and are thus suitable for Lucas hydraulic pumps, which are fitted with components coated with silver and cadmium.  Silver and cadmium do not tolerate zinc.


Hydraulic fluids containing antiwear additives are similar in  all other respects to conventional  standard hydraulic fluids. They are suitable for systems with maximum bulk fluid operating temperatures up to 80°C and at pressures up to 10,000psi (700 bar).

High Performance Antiwear

These fluids contain antiwear additive plus VI (Viscosity Index) improvers, which provide greater viscosity stability over a wider range of operating temperatures. They are suitable for systems with maximum bulk fluid operating temperatures up to 95°C.

Continuous Operation

Continuous operation at the above temperatures may reduce the service life of the oil. Caution should also be exercised with lighter grades e.g. an ISO32 viscosity oil may be more prone to oxidation at continuous high operating temperatures than say an ISO68 viscosity oil. It should be remembered that for mineral based fluids, every 10°C rise in bulk fluid operating temperature above about 80 – 90°C, will result in a doubling of the oxidation rate and consequentially a halving of the oil’s life expectancy.


Conventional mineral based hydraulic fluids are the most widely used in hydraulic oil applications.  They are used in the hydraulic systems of earthmoving, standard injection moulding, road transport, production machinery, agricultural, heavy industry, cranes, fork lift, industrial presses and any other hydraulic equipment that does not, for fire hazard reasons, use fire resistant hydraulic fluids.

This information has been sourced, reviewed and adapted from materials provided by Castrol.

For more information on this source, please visit Castrol.


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