How important is Zinc AFTER break-in?

I do. In everything. Especially flat tappet and stamped rocker stuff. Its worth the extra few bucks.

Hack

 

People forget that most 20's, 30's and 40's flathead engines lived most their life before zinc in oil became 'standard'. Not saying it isn't helpful, but I would say its less important on a low stressed engine with a non-high lift cam.

 

Chevron DELO oil, ZDDP additive and a WIX filter seem to be a good combo, along with 3000 mile change intervals with regular driving.

 

I run Shell Rotella in my older engines with no additional additives.

So far, so good.