Overtightening the thin metal valve cover is a typical source of leaking.
Using spreaders help.
I straighten my valve cover before using a new gasket.
It is the nature of real cork to oil soak a little in order to seal.

One of the most discussed topics:
https://www.stovebolt.com/ubbthreads/ubbthreads.php/topics/1078707

Silicone instead of cork:
https://realgaskets.com/product/csc1-side-cover-gasket-6-cylinder/

"Adding CFM to a truck will only help at engine speeds you don't want to use."
"I found there was nothing to gain beyond 400 CFM."