* Fuel dilution: Unburnt fuel mixing with the oil. This is common if there are problems with fuel injectors, fuel pressure regulators, or the engine's overall combustion process (e.g., worn piston rings). The gasoline thins the oil initially, but over time, it can break down and create a sludge-like consistency.
* Coolant contamination: Leaking coolant mixing with the oil. This changes the oil's viscosity dramatically and creates a milky or muddy appearance. It often smells sweet. This is a serious issue requiring immediate attention.
* Sludge buildup: A buildup of oil contaminants (soot, metal particles, unburnt fuel residue) that create a thick, pasty substance. This is usually the result of prolonged oil changes, using the wrong oil viscosity, or engine overheating.
* Very high mileage and neglected maintenance: Consistent neglect of oil changes can lead to all of the above problems culminating in a peanut-butter-like substance.
Regardless of the specific cause, engine oil that looks like peanut butter indicates severe engine damage or impending damage. Driving with oil in this condition will likely cause catastrophic engine failure. The engine needs immediate inspection and repair by a qualified mechanic. Do not drive the vehicle until it's been thoroughly checked.
