I run the lucas muscle car and classic car oil. It states its 2400 ppm. I have heard ppl on here say that is way to much. Idk Imo 1200 ppm should be 0k. That to me looks like a heat issue not a zinc issue. What kind of oil pressure are you getting? Maybe the 15w50 is to thick and its to dry on the top of the engine. Depending on your bearing clearance. If you have a tight bearing clearance and you are running to thick of an oil you could be starving the top of the engine.