Vacuum pump oil performs the following important functions:

	It lubricates the moving parts and bearings, thus reducing friction, wear, and power requirements.
	It transfers heat from the pump.
	It forms a layer on the inner surfaces of the pump which protects the metal from corrosion. If the concentration of corrosives in the oil becomes too high, corrosives displace the oil film and chemically attack the metal surfaces.
	In mechanical pumps, it seals the small clearances between the moving surfaces in the pumping chamber.
	It flushes away deposits, moisture, and wear debris.

What properties are important in selecting a vacuum pump oil?

Vapor Pressure is the pressure resulting from the evaporation of a fluid in a sealed container. The higher the vapor pressure, the more easily a fluid will evaporate. The vapor pressure increases as the temperature rises. A low vapor pressure is desirable for a vacuum pump oil because it allows you to reach reach low working pressures, minimizes backstreaming, and reduces pump-down time. The vapor pressure of the oil is more important for pumps with lower ultimate pressure such as a TRIVAC dual-stage pump than it is for a pump that operates in the rough vacuum range such as SOGEVAC pump. An oil with high vapour pressure results in poor working pressures and in backstreaming of vapours into the intake line.  Vapour pressure increases as the oil becomes contaminated.

Viscosity is the resistance to flow of a fluid at a given temperature. Oils are more viscous at low temperatures. The viscosity usually increases as the oil becomes oxidized. The oil viscosity needed for each pump model depends on the clearance between moving parts, rotation speed, and operating temperature of the pump.