Probably not relevant , but the first thing that popped into my mind , not for flat tappet applications .
Rat407 said:...Well, since the beginning of time (almost) piston aircraft engines, the closest match being Shell Aero (and others)15/50 and 20/50 oils have no Zinc/Phosphorus/moly or any metallic type of antiwear agents.
The problem is these antiwear agents lead to carbon deposits.
You might want to check your sources.
There is very little to no ZDDP in piston aircraft engine oils but Phosphorus is definitely in there as an anti-wear agent.
The main deposit maker in aircraft piston engines is the lead from the Tetraethyl lead additive in those fuels, and secondarily, carbon deposits from partially combusted oil.