Oxidation is usually used as a synonym for classic oil aging.

Oils oxidize under the influence of heat and oxygen. Acids and oil-insoluble components are formed. These in turn are often the cause of varnish-like resin formation or sludge-like deposits. Oxidation is retarded by anti-oxidants added to the oil by the manufacturer. These agents neutralize the oxygenated compounds that accelerate oxidation and render the reactive molecules harmless.

In addition, these additives deactivate catalytically acting wear particles. However, once the additives are used up, the aging process of the oil accelerates almost unchecked.