Crankcase The oil has to serve two purposes: Reduce friction Cool bearings A good quality mineral oil will serve the purpose of reducing friction to an acceptable level depending upon the metals involved and other conditions such as temperature. All oils will oxidise and this reduces its effectiveness as a lubricant. Oxidation will also cause deposits which can block passage ways and coat working parts. The rate of oxidation will depend upon temperature, the higher the temperature the more rapid the rate. Anti oxidants are available which reduce the rate, also additional properties can be achieved by the use of additives.

Under high temperatures an oil is liable to thermal degradation which causes discoloration and changes the viscosity. Additives cannot change an oils susceptibility to this degradation. The refining process can remove compounds which effect the thermal stability of the oil and also those that lower oxidation resistance. Most of the chemicals found in an oil will react more or less with oxygen, The effects of this oxidation is always undesirable. Hence, a major objective of the refining process of a mineral oil is to remove those hydrocarbons i.e. the aromatics, the small amount of unsaturates together with molecules containing sulphur, oxygen and nitrogen.

Unfortunately these same molecules are those that improve the boundary lubrication performance. Hence, a careful balance must be struck. The use of anti-oxidants make a slightly better balance although there usefulness is limited.

Tin based whitemetal is susceptible to hardening as an oxide layers from on the surface.

These tin oxides are a grey-black in appearance and are extremely hard. There formation reduces the bearing clearance as the oxide layer is thicker than the original white metal material from which it formed. The oxide has a lower coefficient of friction than the original white metal but it will cause problems if it brakes up as fragments will become embedded edge on in the white metal and can score the pin.

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