How Does Motor Oil Become Contaminated? Written by John Baker. Mar 17, 2015 5:25:47 PM from AMSOIL Inc Blog Motor oil deteriorates and becomes unfit for service due to accumulation of contaminants in the oil and chemical changes (additive depletion and oxidation) in the oil itself. Abrasives • Dust and Dirt The design limitations of air cleaners, some oil fill caps and crankcase ventilation systems allow a certain level of dust and dirt into the engine, while leaks in the intake system can permit unfiltered air to enter the engine. Proper maintenance of the engine and its accessories can minimize the amount of contaminants entering the lubrication system and extend engine life. • Metal Particles Normal wear of engine parts produces very small metal particles that are picked up and circulated by the oil. Particles of road dust and dirt increase wear rates and generate larger, even more abrasive metal particles that are circulated through the engine by the oil. While ...