Oil sludge

Oil sludge or black sludge is a solid or gel in motor oil caused by the oil gelling or solidifying, usually at temperatures higher than 100 degrees celsius/212 degrees fahrenheit. 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.
Causes.
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.
Precautions.
Ways to minimize sludge production and accumulation include 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, Volkswagen, Saab, Lexus, and Toyota models are commonly known for oil sludge problems if engines are not serviced as per respective manufacturer specifications.