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In engines, there is some exposure of the oil to products of internal combustion, and microscopic coke particles from black soot accumulate in the oil during operation. Also the rubbing of metal engine parts produces some microscopic metallic particles from the wearing of the surfaces. Such particles could circulate in the oil and grind against the part surfaces causing wear. The oil filter removes many of the particles and sludge, but eventually the oil filter can become clogged, if used for extremely long periods.
The motor oil and especially the additives also undergo thermal and mechanical degradation, which reduce the viscosity and reserve alkalinity of the oil. At reduced viscosity, the oil is not as capable of lubricating the engine, thus increasing wear and the chance of overheating. Reserve alkalinity is the ability of the oil to resist formation of acids. Should the reserve alkalinity decline to zero, those acids form and corrode the engine.