Choosing between engine oil meant for diesel-powered engines and gas-powered engines seems fairly harmless. So long as it's the same viscosity, one might think you can run diesel oil in your normal gas car without issue. Well, you shouldn't. It could actually cause more wear to your engine, according to oil expert Lake Speed Jr.
Viscosity isn't the only thing that differentiates oil. Manufacturers also mix in anti-wear and detergent additives to separate themselves from other brands, and further improve the oil's effectiveness. The main difference between oil meant for diesel engines and gas engines is the additive package mixture.
In a recent YouTube video, tribologist Lake Speed Jr. explains that diesel oil has twice the amount of detergent additives found in oil meant for normal gas engines. That detergent is necessary, as diesel engines tend to inject more particulates, like soot, into their oil during operation.
More detergent might sound like a good thing, but for gas engines, it's a compromise. Speed Jr. says the detergent has to compete with anti-wear additives in the oil to be effective. Diesel engines are designed to work with this compromise, but gas engines aren't. So if you run a diesel oil in a gas engines, you're not giving it all of the anti-wear additives it needs.
It's not just theory Speed Jr. is using as a base for his conclusions. Back when he worked for Driven Racing Oil, he compared four oils of similar viscosity—two meant for diesel engines, and two meant for gas engines—finding the diesel oils resulted in far more wear on the test parts versus the normal oil.
Obviously, not running the proper oil with the proper additives will lead to more wear over time, according to Speed Jr. So as tempting as it may be to run Rotella in your Subaru, we suggest reconsidering.
