SLIPPERY SITUATION

Is diesel motor oil the right choice for a gasoline engine?

- LAKE SPEED JR.

It is widely known that diesel engine oils contain more zinc (ZDDP) than regular passenger car 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 referring to the compound zinc dialkyl dithiophosphate. 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 antioxidant. 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 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 diesel engine oil is not the same as the ZDDP in 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. 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).