[rotorwrench]

The higher the spring pressures on the valves, the higher the need for EP additive. Manufacturers slacked off spring pressures a lot after 1973. It's mostly the high performance hobby and racing that need it now days.

ZDDP only goes so far. 1,200 PPM is about maximum effective amount. We use TCP (tricresyl phosphate) in the Lycoming aircraft engines and it works about as good as ZDDP but it's a bit more toxic.