Oil sludge or black sludge is a gel-like or semi-solid deposit inside an internal combustion engine, that can create a catastrophic buildup. It is often the result of contaminated engine oil and occurs when moisture and/or high heat is introduced to engine oil.
Oil sludge may occur due to a variety of different factors. Some of the most common causes are:
Oil sludge is generally preventable through frequent oil changes at manufacturer specified intervals, however, while uncommon, some engines do have a tendency to build up more sludge than others.