So former Shell Oil engineer Ben Visser's article in July 20 2017 General Aviation News page 12
leads to the conclusion that
a bit less than
1400 ppm of ZDDP
is optimum for wear.

But his comments on non-zinc detergents in some automotive oils burning up and leaving ash deposits that cause pre-ignition pinging also trigger some thoughts.

Which automotive rated oils have the lowest levels of ash deposits?