When a lubricant is exposed to oxygen, oxidation, or the gradual degradation of the lubricant occurs. This is an inevitable process as the lubricant is used, but oxidation is further accelerated if a lubricant is subject to,
a.) High temperatures
b.) Moisture, and,
c.) Catalysts (copper for example)
Typically, a lubricant will oxidize until such time that it is no longer usable, and therefore, subject to a “change-oil”. Symptoms of oxidation will include,
1.) a higher viscosity,
2.) darkening of color,
3.) increase in acidity, and,
4.) the formation of carbon deposits or sludge
Synthetic lubricants, such as Poly Alpha Olefins (PAO) and Esters, are normally more oxidation resistant than mineral-based oils, and hence, can perform as a lubricant for a longer duration. Anti-oxidation additives, on the other hand, are blended with base oils, both mineral-based and synthetics, to increase oxidation stability or oil-life.
Tests such as the Rotating Pressure Vessel Oxidation Test (RPVOT) may be performed to predict the oxidation resistance of various lubricants. A lubricant is placed in a chamber with a copper catalyst, and vacuumed, after which pure oxygen is blown through the chamber. Natural and blended anti-oxidants in the lubricant will resist the exposure to pure oxygen, but once depleted, the oil will react with oxygen, and the pressure in the vessel will eventually drop. The number of hours it takes to reach the pressure drop is then recorded. Oxidation rate, measured in number of hours, becomes useful in comparing the oxidation stability of various lubricants, and can be a criterion in lubricant selection.