The Zinc wear addiative ZDP had to be removed from oil because it fouled the dual bed catalitic converters, around 1995 is when they took it out. I just use Shell Rottella, it is available everywhere and has ZDP, also carries a SM API rating for gasoline engines. They just changed the formulation, but it still has ZDP, I emailed Shell. Or you can use racing oil, additives every oil change, etc.