The Story Of Why Engine Oil Becomes Engine Sludge, Don Fedak, Engine Builder, September 2001

Fresh engine oil is a clear, free-flowing liquid blend of base stock and additives that contains no fuel, water, coolant, dirt, or other contaminants. In engines that have failed prematurely, the oil has very often been transformed into a high viscosity deposit of brown or black goo, commonly referred to as "sludge."

When regular engine oil changes are neglected, normally free-flowing lubricating oil breaks down, becomes contaminated, ceases to flow, and is transformed into a thick soup of waste products. That´s when serious engine damage is imminent.

Why does engine oil break down, combine with contaminants, and form sludge deposits? Chemistry teaches that engine oil is unstable and decomposes in the presence of oxygen at high temperature. The process, called oxidation, occurs naturally after exposure to normal operating conditions for extended periods of time and is accelerated by exposure to severe operating conditions or to excessively high temperatures. Alternatively, accelerated oxidation may be triggered by a combination of any or all of these factors.

During oxidation, the chemical bonds that define the oil molecules are broken, and some of the reaction products accumulate and interact to form a highly viscous complex mixture of solids, liquids, and gases that contain a variety of solid carbon-based dirt and metallic particles, as well as liquid coolant, fuel, oil and water droplets.