Zinc Rich Oil for Flat Tappet Cams

318willrun said

Just my opinion, too much zinc has it's down sides as well. Spring pressure and engine use plays a part, but in a average street engine 1300 -1500 ppm serves just fine. That's about what the oil had back in the day. The rest of todays oils are better - minus the zinc.

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Interesting bit of history is that ZDDP's first purpose was to reduce oxidation. See the Grant article linked above. In the 70s they started bumping the ZDDP to .1% so the oil wouldn't thicken up. And if Grant's timeline is correct, it would be from 1980-88 that most oils had 1200 - 1500 ppm (API SF). Then levels started dropping, especially for oils meeting ILSAC GF specs.