According to Lake Speed, Jr. of Joe Gibbs Driven Racing Oil, one of the important differences between racing oil and API oil is the limited amount of phosphorous in API blends. The EPA limits the amount of phosphorous and zinc, specifically it’s the phosphorous, not the zinc that is limited. Phosphorous is a component of Zinc dialkyl dithio phosphate (ZDDP, or ZDP) is a family of zinc salts of dithio organophosphates. And they easily dissolve in mineral and synthetic oils that are used as lubricants.
Zinc phosphate is mainly for anti-wear. The zinc and phosphate go hand and hand. So when you limit phosphate you limit zinc as well.
“You can put zinc in the oil but it won’t act as an anti-wear agent until you add the phosphorous,” says Speed. “When you have a limit on the amount of zinc-phosphorous you limit the anti-wear agents. The combination of zinc and phosphorous is limited to a maximum of 800 parts per million in API/SM classification, which is the latest classification that came out in 2004.
“Any oil that you get that says API or ILSAC GF-4 contains a limited amount of zinc and phosphorous, and for most racing applications, that’s not what you want,” says Speed.
It is for this reason some engine builders have switched to diesel oil for break-ins as some of these formulations are known to have higher levels of zinc.
“The current diesel oil spec is CJ-4 is limited to 1,200 parts per million (ppm), so it does have 400 ppm more than API pass car oil but it’s only marginally better. A lot of people get mislead hunting down the oil with the highest ppm, as if it were the Holy Grail, and it really isn’t,” says Speed.