Oil sludge or black sludge is a solid or gel in motor oil caused by the oil gelling or solidifying, usually at temperatures lower than 100 degrees Celsius. Oil sludge can be a major contributor to internal combustion engine problems, and can require the engine to be replaced, if the damage is severe.
Sludge is usually caused by a poorly designed or defective crankcase ventilation system, low engine operating temperatures, the presence of water in the oil or crankshaft-induced cavitation, and can accumulate with use. Oil sludge that causes an engine to run poorly or even seize is a rare occurrence and typically happens only when oil changes have been neglected or if the car has been driven extensively with an extremely low oil level.
Ways to minimize sludge production and accumulation includes performing frequent oil changes, performing mechanized engine flushing, or de-sludging, using synthetic oil, and following the manufacturer's engine maintenance routine. Audi, Chrysler, Dodge, Lexus, Volkswagen, Saab and Toyota, models are known for oil sludge problem, if engines are not serviced as per manufacturers specifications.