A Reader Asks: Is it OK to use a motor oil that is meant for a gasoline car engine in a diesel motor? If the diesel engine calls for 10W-30 engine oil, can most any good 10W-30 be used?

Our Answer: In most cases, you should not use a motor oil designed for gasoline engines or passenger car service in a diesel engine. The exception would be motor oils that carry the correct diesel oil specification as suggested by the diesel engine manufacturer (more on that in a bit). A common mistake we see people make is that their diesel engine calls for a 10W-30 engine oil and they assume that any 10W-30 will suffice. This is definitely not the case. We carry some of the best synthetic oils for passenger car/gasoline engine service on the planet and would never recommend that these products be used in a heavy-duty diesel engine.

The reason being is that the combustion that takes place in diesel engines generates a significant amount of soot that is not found in gasoline engines. Soot is an abrasive carbon substance that results from incomplete diesel fuel combustion. Heavy-duty diesel engine oils are fortified with additives that hold the soot in suspension and strong detergents to keep engine internals clean.

Also, diesel engine oils tend to have a higher total base number or “TBN” than gasoline engine oils. This TBN number represents the motor oil’s ability to counteract acids that can build up in the motor oil over the service life. These acids are also a result of the combustion process.

What Would Happen If You Used Gasoline Engine Oil In A Diesel Engine?

It would not be an instant disaster, but over time the oil would become loaded with soot and would not have the capacity to deal with it. Deposits will start to form in the engine and would be especially severe in the upper valve train area. Eventually excessive engine wear would start to occur.

Can You Use Diesel Engine Oil In a Gasoline Engine?

Provided that it meets the diesel motor oil meets the correct motor oil specification for your vehicle, diesel engine oils will function perfectly in a gasoline engine. If the specification matches, there is no downside to using a diesel engine oil in a gasoline engine.

Engine Oil Specifications Explained

We’ve mentioned motor oil specifications several times, so let’s briefly touch on this important component of the discussion. The manufacturer of every modern gasoline or diesel-powered vehicle or piece of equipment will prescribe that the motor oil used in the engine meet a particular industry specification. These specs are always marked on the label and should be noted when choosing a motor oil. Here are some examples of API (American Petroleum Institute) motor oil specifications.

Gasoline Engine/Passenger Car Service

  • API SN (newest specification)
  • API SM (previous specification)
  • API SL (previous specification)
  • API SJ (previous specification)

Diesel Engine Service

  • API CJ-4 (newest specification)
  • API CI-4 Plus (previous specification)
  • API CI-4 (previous specification)

Most motor oils are formulated to cover multiple specifications, so take special note to make sure that spec required for your vehicle is on the label. European car makers usually have their own unique motor oil specifications which should be observed.

Bottom line: Do not fixate only on the viscosity or grade when choosing a motor oil. Make sure that the correct industry specification is on the label when shopping for motor oil and you won’t go wrong.