The lubricants commonly employed are refined from crude oil after the fuels have been removed. The best engine lubricant viscosities must be appropriate for each engine, and the oil must be suitable for the severity of the operating conditions.
Oils are improved with additives that reduce oxidation, inhibit corrosion, and act as detergents to disperse deposit-forming gums and solid contaminants. Motor oils also include an antifoaming agent.
Various systems of numbers are used to designate oil viscosity; the lower the number, the lighter the body of the oil. Viscosity must be chosen to match the flow rate of oil through a part to the designed cooling requirements of the part.
If the oil is too thick it will not flow through the part fast enough to properly dissipate heat. Certain oils contain additives that oppose their change in viscosity between winter and summer.
The lubrication system is fed by the oil sump that forms the lower enclosure of the engine. Oil is taken from the sump by a pump, usually of the gear type, and is passed through a filter and delivered under pressure to a system of passages or channels drilled through the engine.