sludge

in gasoline engines, a black emulsion of water, other combustion by-products, and oil formed primarily during low-temperature engine operation. Sludge is typically soft, but can be very hard. It plugs oil lines and screens, and accelerates wear of engine parts. Sludge deposits can be controlled with a dispersant additive that keeps the sludge constituents finely suspended in the oil.

Also see engine deposits.

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