Fel Pro cork for me. Cheap and affective.
I get the bottom of the valve cover nice and straight, and hospital clean. Get out my Gasgacinch, and glue the cork gasket to the valve cover. Then I set it on a flat bench with some weight on top, and let it dry. After they dry, put them on, and torque to spec.
Through my experience, the oil gets out between the gasket and the valve cover, not the gasket and cylinder head. Its also, usually at the bottom of the valve cover.
Ahh sounds reasonable, yeah I glued these on, they have a ridge around the sealing surface...ever see that, I bet the cork is softer and will seal better. I'm gonna try a 1/4" 20 tomorrow and see if I can get it tighter than the T nuts