Is Diesel Motor Oil the Right Choice for a Gasoline Engine?

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It is widely known that diesel engine oils contain more Zinc (ZDDP) than regular passenger motor oils, but does that make a diesel engine oil (HD) a good choice for hot rods or race cars? To answer that question, we need to understand the other differences between diesel oils, passenger car oils, and high-performance oils.

Let's talk about Zinc. When we say Zinc in reference to motor oil, we are actually referring to the compound Zinc DialkylDithioPhosphates. That's hard to say or spell, so we just call it Zinc or ZDDP. Either way, it is a powerful anti-wear additive and anti-oxidant. However, not all ZDDP is the same. What? Yep, those brilliant chemists at the oil companies can fine-tune the way ZDDP works in the engine. That might sound weird, but ZDDP works sacrificially—ZDDP actually sacrifices itself to protect your engine—and by tweaking the chemistry behind it, the conditions required for the ZDDP to activate can change.

Why does this matter? Because the type of ZDDP in a diesel engine oil is not the same as the ZDDP in a gasoline engine oil. This is an important difference; diesel engines run at lower rpm than gasoline engines, so that difference in engine speed changes the way the ZDDP needs to activate to provide anti-wear protection. To get a little technical, diesel engine oils typically contain primary ZDDP and gasoline engine oils contain a blend of secondary and primary ZDDP. Just as a side note, racing oils just contain secondary ZDDP. Both types of ZDDP offer anti-wear protection, but the gasoline type of ZDDP provides more protection for higher-speed engines (think bigger valvesprings).

So more ZDDP isn't better? That's right. Just adding more ZDDP to an oil doesn't mean it will provide better wear protection. In fact, too much ZDDP can increase wear. You need both the right kind of ZDDP for the engine type and the right amount of that ZDDP. It's like Goldilocks and the Three Bears: not too much, not too little, just right. And in case you didn't know, some diesel oils don't have more ZDDP than gasoline engine oils (also known as Passenger Car Motor Oil, or PCMO). So, if the type and amount of ZDDP in a diesel oil is not ideal for a high-performance gasoline engine, is that the only reason not to use a diesel oil in a gasoline engine?

Going back to engine speed, there are other additives in the oil tailored to the lower rpm of diesel engines. The first that come to mind are anti-foam additives. The churning of the motor oil in the crankcase by the motion of the crankshaft, connecting rods, and pistons creates a frothy mess. To combat that, anti-foam additives are blended into the oil. The type and amount of anti-foam additives in an oil varies by the type of oil. You've probably already guessed it, but yes, diesel engine oils have a different amount of anti-foam additive. For example, a typical diesel oil formula might have less than 5 parts-per-million of anti-foam additive. A racing oil designed for higher-rpm engines will have up to 10 parts-per-million of anti-foam additive. That's a significant difference for this type of additive, and it can have a significant impact on your engine's health and performance.

Air is a terrible lubricant, so aerated oil can lead to bearing failure and a host of other problems. Today's variable-valve-timing and displacement-on-demand engines rely on the engine oil to function as a hydraulic fluid as well as a lubricant. Aerated oil is compressible, so it can negatively affect the function of these systems.

But wait, there's more! Diesel engine oils contain high levels of detergent and dispersant additives because diesel combustion creates a much dirtier environment. If you have a gasoline direct injection engine, pay special attention to this part. The high levels of calcium detergent additives in most diesel engine oils have been proven to cause detonation in direct-injection gasoline engines. This even has a special name: low-speed preignition (LSPI). The current API SP gasoline engine oil standard reduces the calcium levels in order to pass a low-speed preignition test. Also, those dispersant additives will hold fuel and moisture in the oil, so a diesel engine oil is not the ideal choice for an engine running on E85, unless you just like your oil to look like a chocolate milkshake …

After 15 years of formulating and testing oils, one thing has become abundantly clear: The application always dictates the chemistry. When choosing an oil, don't start with the oil. Instead, start by defining the application—gas or diesel, boosted or NA, drag race or street, etc. Let the application determine the type of chemistry needed. Here's a hint: If your car goes to the racetrack on a trailer, you should be using a racing oil. The actual brand of oil is the last choice. Let the application dictate the chemistry of the oil and then decide which brand delivers that chemistry. You don't need to be an oil formulator to figure this out. Trust me, the tech guys at the oil companies love it when customers come to them with all the details of the application and ask for their recommendation. Get all your information together before contacting a few different brands available through your supply channels. Things like bearing clearance and operating oil temperature are important when determining the right viscosity oil to use. That's right, 20W-50 is not a one-size-fits-all choice (but we will save that for another article). In the meantime, you know more about oil chemistry, and you have some homework to do. At least it's related to something you love.

Until next time, happy hot-rodding!

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