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.

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Oil sludge may occur due to a variety of different factors. Some of the most common causes are:

• Defective crankcase ventilation system
• Oil/coolant contamination
• Neglecting oil changes
• Low oil level
• Poor engine design

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.