I’ve been instructing the Fundamentals of Machinery Lubrication course for a few years now, and many times the course has been the birthplace of the articles I’ve written. The questions and discussions in the course are very representative of what the average maintenance professional is curious about. Most tend to like the discussions about passenger car and truck oils because they not only affect them at work but also at home. In fact, I use this curiosity to my advantage. If I ever notice a subject is starting to bore an audience, I’ll find a way to incorporate an automotive twist to it. Soon all the ears seem to perk up. This article is no different. I plan to explore the similarities and differences between gasoline and diesel engine oils to appease your curiosity.

In the broadest sense, gas and diesel engine oils have the same anatomy or makeup. They are formulated from the blending of base oils and additives to achieve a set of desired performance characteristics. From this simple definition, we start to diverge when examining the lubricant’s required performance for each engine type.

Emissions and the Catalytic Converter

A catalytic converter is a housing that contains porous metal filler located between the engine and muffler in the exhaust system. Its role is to convert toxic emissions coming from the engine to stable byproducts before they enter the atmosphere. Some of the byproducts of combustion (lead, zinc and phosphorus) can severely cripple the converter’s ability to perform this job. Therein lies the first major difference between the oils. Diesel engine oils have a higher anti-wear (AW) load in the form of zinc dialkyldithiophosphate (ZDDP). The catalytic converters in diesel systems are designed to be able to deal with this problem, while the gasoline systems are not. This is one of the main reasons you don’t want to use a diesel engine oil in your gasoline engine. If your automobile was built prior to 1975, there is a good chance it does not have a catalytic converter, and thus the above statements do not apply.

Viscosity

Viscosity is the single most important property of a lubricant. When I am working as a consultant and designing a lubrication program, one of the first steps I take is to calculate required viscosities. Getting the right viscosity is of the utmost importance. The selected viscosity needs to be pumpable at the lowest start-up temperature while still protecting the components at in-service temperatures.

Typically, diesel engine oil will have a higher viscosity. If we were to put this higher viscosity in a gasoline engine, several problems might arise. The first is heat generation from internal fluid friction. I’ve covered before how this heat affects the life of an oil. A good rule of thumb is that for every 10 degrees C you increase the temperature, you cut the life in half. The second problem is the low-temperature pumpability of this higher viscosity. During cold starts, the oil may be very thick and difficult for the oil pump to deliver to the vital engine components in the lifter valley. This most certainly will lead to premature wear, as the components will be interacting without the benefit of lubrication.

Additive Levels

Diesel engine oil has more additives per volume. The most prevalent are overbase detergent additives. This additive has several jobs, but the main ones are to neutralize acids and clean. Diesel engines create a great deal more soot and combustion byproducts. Through blow-by, these find their way into the crankcase, forcing the oil to deal with them. When you put this extra additive load in a gasoline engine, the effects can be devastating to performance. The detergent will work as it is designed and try to clean the cylinder walls. This can have an adverse effect on the seal between the rings and liner, resulting in lost compression and efficiency.

So how do you know if an oil has been designed for gasoline or diesel engines? When reading a label, look for the API (American Petroleum Institute) doughnut. In the top section of this doughnut will be a service designation. This designation will either start with an “S” (service or spark ignition) for gasoline engines or a “C” (commercial or compression ignition) for diesel engines.