After a period of use, motor oil deteriorates and becomes unable to do an efficient job lubricating and cooling engine parts. The two main reasons for this are:
Accumulation of contaminants
During the normal operation of an engine, contaminants begin to accumulate in the engine oil. The engine oil is designed to lock away and neutralize by-products of combustion such as soot, carbon and acids. As contaminant levels build-up over time, critical properties of the engine oil such as viscosity can be affected.
Depletion of oil additives
Additives are consumed as they perform their vital role. If the additives are depleted, corrosion, oxidation and accelerated engine wear can occur. For this reason, engine manufacturers recommend that the oil be changed at certain time or distance intervals to ensure protection of the engine.