Oil sludge

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.

Causes
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

Precautions
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.