Oxidation

A form of chemical deterioration to which all petroleum products are subject; the addition of oxygen atoms results in degradation. It is accelerated by temperatures above 160°F (71°C), with the rate of oxidation doubling by each 20° increase. With fuels and lubricant oils, oxidation produces sludges, varnishes, gums, and acids, all of which are undesirable. Oxidation also leads to increasing oil viscosity and oil darkening.