Yep, ZDDP is for the exhaust valve seat to replace the lead that was in fuel. Avgas is for high compression engines. The 8n is not.. MMO is a good lubricant, but don't see it helping a valve seat.
No, ZDDP is for the oil, and it cushions the lobes of the cam, the mating surface of the lifters, etc..
Folks should add it to the oil for ANYTHING running a flat tappet cam. The new SN oils don’t have enough zinc, anymore.
https://www.google.com/amp/s/www.sp...at-is-zddp-benefits-of-zinc-in-oil/30870
