Diesel and gasoline engines have different needs when it comes to oil.

Your diesel fires by compression, without a spark to ignite the fuel. The fuel-air mixture is compressed until it gets so hot that it fires on its own. This means higher temperatures, which can break down lube oil designed for a gasoline engines into its basic components of hydrogen and carbon. It will then burn it off, a process that promotes the buildup of carbon deposits in the engine, which increases wear on engine surfaces.

The solution is to use oil that is properly formulated for marine diesels. The difference is in the additives. Lube oils approved for diesel engines – one popular brand is Chevron Delo 400 Multigrade – are designed to operate at higher temperatures, and have properties designed to counteract the byproducts of combustion.

Diesel fuel is less refined than gasoline and contains heavy chemicals removed by the gasoline refining process. These concentrated chemicals form several corrosive compounds during combustion, which eat away at your engine when it sits unused for weeks at a time. The additives in proper diesel lube oil neutralize them.

In reality, your diesel engine may seem to run fine for many years using automotive lube oils, and you are unlikely to notice a difference in performance. But it is deteriorating faster than it must. If you plan to nurse that engine along for decades, you will want to use proper diesel engine oil.

In fact, many professional mariners use diesel-grade oil for both gasoline and diesel engines. You can use diesel-grade oil in a gas engine, but you should only use diesel-grade oils in a diesel engine except in a true emergency when a diesel grade is not available.