Over the last 30 yrs. of running Mopar big blocks, I've only had cork gaskets work for me consistently. I've had trouble sealing all the 1/2-dozen other types I've tried. I do like to use a little adhesive/RTV to hold them in place on the valve covers during installation, but not necessary (just keeps gasket from slipping and keeps holes lined up).

70 Roadrunner convt. street car 440+6, NOS, 4-spd, SS springs '96 Mustang GT convt. street car '04 4.6 SOHC, NOS, auto, lowered "Officer, that button is for short on-ramps"