"Oil sludge" refers to a thick and sticky substance that forms when oil breaks down or gets contaminated. It can clog up engines and pipes, causing damage and reducing efficiency. Full definition
They are caked with oil sludge and carbon build up. (humblemechanic.com)
What's the preferred, non - chemical / non-additive method for removing oil sludge from an engine that's still installed in the vehicle? (mechanics.stackexchange.com)