The maximum amount of anti-wear additive (ZDDP) is capped by API/ILSAC depending on viscosity and the API/ILSAC rating of the oil. In general, the API SM oils are going to be capped at 800 parts per million (ppm), while the older API SL allows up to 1,100 ppm. Most of the Royal Purple engine oils have retained an API SL rating and they generally treat at or near the allowable level. The limit for viscosity grade of 10W-40, 15W-40 and 20W-50 have a higher allowable level of 1,200 –1,300 ppm. Since the Royal Purple XPR is a racing oil they do not have to hold to the goverment standards for Maximum ZDDP
