What Kind of Oil Should Be Used in a Diesel Engine?

by Will Chandler

Diesel engines need a high viscosity oil or a synthetic oil . Since these engines combust at much higher compression ratio than gasoline engines, the oil used in the motor has to be able to withstand tremendous forces, usually of 40,000 to 50,000 psi. Most newer diesel engines will recommend 15W-40 because it is heavier and thicker.


Oil is used to lubricate the pistons as these are pushed into the combustion chamber and return downward to the crankcase. Diesel engines typically turn at a lower rate than a gasoline motor, but create greater pressure. Heavier oils last longer and can withstand the workload created by the diesel motor.


Synthetic oils can also be used in a diesel motor. 5W-40 synthetic can be used in most newer motors with equivalent or better results than a regular 15W-40 oil. Synthetic oils are more expensive, but provide additional protection against motor wear and tear.

Oil Changes

Synthetic oils still have to be changed. Follow the car manufacturer's recommended mileage for changing the oil. Always remember to change the oil filter as well as the engine oil. Most diesel motors recommend changing the oil every 5,000 miles.

Benefits of synthetic oil

Synthetic oils can lower the exhaust gas temperatures(EGT) in a diesel by a significant amount. Lower EGT means the engine can operate at maximum efficiency and create less heat, which can reduce damage and wear to vital motor parts. A normal operating temperature for a diesel should be around 500 degrees, rising to about 1000 degrees under acceleration.


A diesel engine is less volatile and burns at a lower temperature than a gas motor. This is the reason that high exhaust temperatures are so dangerous. The turbo on a diesel motor filters air and compresses it before forcing it into the combustion chamber to mix with diesel fuel and power the motor.

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