Rubber gaskets always leak on Pontiac engines. The factory covers are too thin and have too much span between fasteners. Not a shining moment in Pontiac design. I have had the best luck by far with cork gaskets. Mr. Gasket sells a really nice set in the Ultraseal line. It has a paper thin coating of black sealer sprayed over the cork so it doesn't wick too much. I tack them in the cover and put nothing on the head side. That's my best recommendation. Anything silicone or rubber leaks for me, I don't use them. Good luck.
https://www.speedwaymotors.com/Mr-Ga...CABEgLXzvD_BwE