Gummy or carbonaceous material that accumulates in lubricating oil is called SLUDGE. Most engine lubricating oils have some natural ability for preventing conditions that may cause sludge to form and for carrying sludge that does form in a finely suspended state until it is removed by filtering equipment. Chemicals are added to some oils to improve the ability of the oils to prevent the formation of sludge. The formation of sludge is greatly reduced when the lubricating oil has the proper stability. STABILITY is defined as the ability of the oil to resist oxidation and deterioration for long periods. Proper stability is essential for a strong oil film to be maintained throughout the normal temperature ranges of an operating engine. A strong oil film provides the required oiliness, or film strength, to form a seal between the piston and the cylinder wall. With the oil seal in place, burned fuel and exhaust gases cannot get by the piston rings to form sludge.

Various factors tend to cause sludge to form in an engine. Carbon from the combustion chambers or from the evaporation of oil on a hot surface, such as the underside of a piston, will cause sludge to form. Gummy, partially burned fuel, which gets past the piston rings, or an emul-sion of lubricating oil and water, which may enter the lubricating oil system, will also tend to form sludge.

Sludge in the lubricating oil system of an engine is harmful for several reasons. In addition to carbon and gummy material, sludge may contain abrasive ingredients, such as dust from the atmosphere; rust as a result of water condensation in the engine; and metallic particles resulting from wear of engine parts. Sludge in engine lubricating oil causes premature wear of parts and eventual breakdown of the engine. Sludge may clog the oil pump screen or collect at the end of the oil passage leading to a bearing, thereby preventing sufficient oil from reaching the parts to be lubricated. Sludge may coat the in-side of the crankcase, act as an insulator, and blanket the heat inside the engine. This condition will cause the oil temperature to increase and induce oxidation. Sludge may accumulate on the underside of the pistons and prevent proper heat transfer, thereby raising piston temperatures. Sludge in lubricating oil also contributes to sticking piston rings, a condition that will affect the ability of the rings to seal the cylinder.