الفهرس 
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Abstract
The theories of lubrication of the different parts of an internal combustion engine are sufficiently understood to enable a suitable lubricating oil to be selected for the lubricating conditions prevailing in these engine in normal use. Thus, it may be said that providing such lubricating oil maintains its properties during use in an engine, the lubrication of modern heavy duty internal combustion engines would be an easy matter.
Unfortunately, any lubricating oil, after use in an internal combustion engine for some time, deteriorates and its properties are no longer the same as when the oil was first put into the engine. The changes which take place in the oil could consist of :
a) A change in the viscosity, and this change is usually towards an increase in the viscosity, except when excessive dilution from fuel takes place. Such increase in the viscosity may reduce the rate of flow of oil through the different engine parts and thus impedes the cooling effect of the oil. Again, the oil with high viscosity will affect the engine performance, especially fro the point of view of ease of starting in cold weathers.