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. 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. 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.