A：The impurities in the oil are roughly classified into two categories. One type is the dust that enters the fuel filler when the engine parts are worn out during operation and the oil is replenished. The other type is organic matter, which is black clay. This is the substance produced by chemical changes in the high temperature state of the engine during engine operation. They degrade the performance of the oil, weaken the lubrication, and stick to moving parts and increase the resistance.
