Is Diesel Engine Oil Any Different From Regular Oil?

Regardless of whether it burns gasoline or diesel, your internal combustion engine works by igniting a mixture of fuel and air to create power. To do so effectively, it must reduce the detrimental byproducts created by its operation, including heat, contaminants, and friction. Both gas and diesel engines use oil to combat these limiting factors, but that does not mean the two types of oil are the same.

Oil, the lifeblood of an engine, is pumped through filters and galleys to journals, bearings, and other mechanical parts to keep them running smoothly. Along with dissipating heat, oil also carries away metal particles, soot, condensation, and other contaminants, helping prevent the buildup of sludge. Its slippery, film-like properties reduce friction between surfaces, helping engine efficiency while protecting it from wear. When an engine has the wrong oil — or no oil– it may wear prematurely or fail completely.

While all ICE engines follow the same principle, different types of internal combustion engines work differently. Putting gas in a diesel engine can cause damage, while diesel in a gas car gets you nowhere fast. Similarly, each engine type should be filled with its own type of oil. A diesel engine typically operates at higher temperatures and pressures than its gas counterpart. It produces more torque, but also more particulates and acidic byproducts than a higher-revving gas engine. For these reasons, diesel engine oil is different to regular oil, not only in its viscosity, but in the amount and the type of additives.

Making engine oil is, by definition, a refined process. Manufacturers create oils of different viscosities for different applications, adding extra substances to perform specific functions. These additives include everything from anti-foaming agents, corrosion inhibitors, to detergents, dispersers, friction modifiers, and zinc dialkyldithiophosphates (ZDDP) – not to mention the wide range of aftermarket oil additives containing everything from stop-leak to snake oil.

The amount and type of oil additives will depend on the engine. For example, burning diesel fuel creates more acidic byproducts than gasoline, so diesel engine oil has a higher total base number (TBN) than regular oil, meaning it has more alkaline or base ingredients available to neutralize the corrosive effects of these acids. On the other hand, high-revving gasoline engines can potentially whip oil into foam, severely limiting its lubricity, but because diesel engines operate at lower rpm, regular engine oil will have more anti-foaming agents than a diesel formulation.

Diesel engines produce more soot and smoke than gas engines – a byproduct of incomplete combustion — which is why it's somewhat normal for diesel engine oil to turn more black than regular oil. As mentioned previously, to deal with these particulates, diesel engine oil has special additives that actively dissolve and suspend contaminants before removing them at the filter, minimizing their deleterious effects on the engine. Regular oil, on the other hand, will require a different set of specialized additives, for example, to prevent contamination of the catalytic converter.

Diesel engines use compression to generate sufficient heat to ignite the fuel without the need for a spark plug. To do this, a diesel engine runs at a higher compression ratio than an equivalent gas engine, producing proportionally more torque and putting more pressure on turning parts. Given the viscosity, or thickness of an oil decreases with temperature, the hot, high-pressure diesel engine requires a higher viscosity oil. 

This doesn't mean a high viscosity oil will offer better protection for any old engine. Oil is scientifically graded for correct viscosity not only when cold, but also under high pressure at operating temperatures, with these two different values represented in the viscosity rating. 

For example, the W in 15W-30 stands for winter, with 15 being the kinematic viscosity at low temperatures, and 30 representing the dynamic figure at operating temperature. The higher the figure, the thicker the oil. It's important to note that the two values are calculated differently, and should be compared like-for-like, and not to one another.

Given 75% of engine wear occurs on start-up, it pays to choose the best oil weight for your diesel engine. While you can find the manufacturer's recommendation in your owner's manual, you also check the American Petroleum Institute's Motor Oil Guide to ensure you have the correct type of oil. A, C, or F prefix means the oil is formulated for diesel engines, while S or G means the oil is for passenger car gas engines.