Most commonly used form of hydraulic fluid ("oil hydraulics"). Their high level of lubricity and relatively high viscosity make it possible to use gap seals in pumps and valves and therefore choose a very compact design (unlike with pressurised water). Having said this, they are also associated with certain disadvantages such as the way their temperature-dependence of viscosity (VT), compressibility or ageing behaviour influence the processes used in hydraulics.
Mineral oils consist of napthaor paraffin base oils and additives designed to improve or even suppress existing characteristics. Since the various base oils affect the sources of elastomer seals in different ways, it is important to ensure the oil and seals are right for each other.
Depending on the additive involved, a distinction is made between HL, HLP, HLPD and HV oils.