Purpose of motor oil (BMW 5 Series E34)

During operation, engine oil is exposed to a wide variety of influences. Therefore, it is very difficult to accurately determine the effect of various operational influences on the engine lubricant. Engines that operate for a long time at high speeds or at full load reach high temperatures. Under the influence of high temperature and atmospheric oxygen, the process of oil oxidation begins. Oxidation products thicken the oil and can form varnish-like deposits on the upper parts of the pistons, in the grooves under the piston rings and on the valve stems. This can lead to coking of the valve heads.