What Oil Weight Do You Need For A Diesel Engine? Viscosity, Explained

The second car that I ever owned was a 2002 Chevy Silverado 2500 with a 6.6L Duramax diesel engine under the hood. It wasn't my first choice, but it was a hand-me-down from my father after my 1993 Volvo 240 bit the dust. I knew that Volvo like the back of my hand. I did all of my own maintenance and memorized all of the basics including how to replace its fuel pump, swap out brake pads, and what oil to use during an oil change. Moving into a 3-ton diesel, most of that information went right out of the window. When I got the truck, it was due for an oil change, so I started my nerdy research right away, learning the difference between gasoline engine oil and diesel engine oil.

I was relieved that I didn't just go ahead and throw in the spare 5W-30 oil that I had lying around from my Volvo, which I seriously had considered, as there are significant differences between conventional and diesel oil. Due to the fact that diesel engines produce more contaminants during the combustion process, diesel oil contains additional additives and detergents to counteract that. Even more importantly, diesel oil is generally more viscous, as the engines need more protection against the increased combustion forces produced by diesel compared to gasoline. For that reason, it is critical to understand the importance of oil viscosity in diesel engines, as it plays a major role in engine health.

The actual concept of oil viscosity is pretty simple. It is simply a measure of how hard it is for the oil to flow. In even simpler terms, it is the measure of how thick or thin the oil is at a given temperature. Oil viscosity is most frequently measured on a scale created by the Society of Automotive Engineers (SAE), which measures dynamic viscosity — the thickness of oil when it is cold — and kinematic viscosity — the thickness of the oil at engine operating temperature. The letter 'W,' which stands for 'winter,' and a dash separate the two measurements when using the scale. For example, a common oil rating is 15W-40, with 15 representing the oil's dynamic viscosity (with the W as a reminder of the cold value) and 30 representing the oil's kinematic viscosity. With both the dynamic and kinematic values, the lower the number, the less viscous, or thinner, the oil is.

Oil viscosity plays a major role in protecting the engine against internal damage. As oil flows through an engine, it coats the engine's moving internal parts with a film, preventing the parts from touching directly, which can cause damage. If the oil is too thin or not viscous enough, the oil might not be able to provide enough protection against the friction caused by metal parts rubbing against one another. Diesel engine oil is typically more viscous than conventional oil, as the increased compression and combustion forces in diesel engines cause oils to break down more easily, becoming less viscous. That can cause significant engine damage over time. There is a balance, though, as there is a point where oil is so thick that it affects engine efficiency.

There is no catch-all engine weight that is the best option for all diesel engines, as all diesel engines are built differently. Diesel engines from one manufacturer could have completely different internal clearances than engines from another manufacturer, which plays a role in determining the best oil viscosity for each. For that reason, it is best to consult the vehicle owner's manual to check the recommended range of approved oil viscosity.

That range will typically be wide enough to accommodate a bit of variation depending on other factors like climate and how the vehicle is being used, which also have an effect on the right oil weight to use. For example, if you are located in a warm climate, higher-viscosity oils are better to use, as the warm environment counteracts the oil's thicker nature and allows it to flow easily through the engine. The opposite is the case for colder climates, where lower-viscosity oils are a better option to promote oil flow.

Regardless of the oil weight, it is also important to use an oil with the proper rating for your diesel engine. This is typically classified with an API (American Petroleum Institute) rating, which indicates whether the oil meets the right performance standards for a particular engine. Oils with an 'S' rating are made for gasoline engines, while oils rated with a 'C' are made for diesel engines. It is an important distinction, as diesel and gas oils use different zinc contents, anti-foaming agents, and detergents which are all specifically tailored to provide the best lubrication, protection, and performance for each type of engine. For that reason, it isn't a good idea to use diesel engine oil in gasoline powered cars.