Depending on the operating conditions, different requirements are imposed on engine oils. Therefore, it is very difficult to accurately determine the various operating conditions in terms of their effect on lubricants. Engines running for long periods of time at high speeds or under full load develop very high oil temperatures. Under the influence of high temperatures and atmospheric oxygen, the process of oil oxidation begins. Oxidation products thicken the oil and can be deposited as paint coatings on piston tops, piston ring grooves and valve stems. This can lead to coking of the valve discs.