The hydraulic system of cars and engineering machinery, etc. use hydraulic oil as working medium. The flowing speed of oil in this type of hydraulic systems is not high, but the pressure is relatively high, so it is called hydrostatic transmission. The quality of hydraulic oil, to a very large extent, will influence the work reliability and service life of hydraulic system. Usually the quality requirements of hydraulic oil are as follows:

1. A proper viscosity and a good viscosity temperature performance to guarantee that the lubricant can correctly and sensitively transmit the dynamic force when the working temperature changes and guarantee normal lubrication of hydraulic components.

2. A good anti-rusting property and a stable anti-oxidation, it's not easy to get oxidized and go bad under a high temperature and a high pressure, with a long service life.

3. A good resistance to foam makes it easy for the foams generated to vanish under the working conditions when the lubricant is continuously stirred by the machinery, so that the transmission of dynamic force is stable and it can prevent hydraulic oil from oxidation at a faster speed.

4. A good demulsibility can quickly separate lubricant from water mixed into the oil, lest emulsion is formed and the metal of hydraulic system is rusted and the using effect is affected.

5. A good extreme pressure anti-wear property can guarantee that the hydraulic oil pump, hydraulic motor, control valve and friction pair of the oil cylinder can be lubricated normally under a high pressure and a high speed and wear will be reduced.

Other than the above-mentioned basic quality requirements, there are some special requirements for some hydraulic oil. Like, low temperature hydraulic oil requires a good low temperature using performance; fire resistant hydraulic oil requires a good fire resistance; silver resistant hydraulic oil can be used in hydraulic system with silver parts.