The stock valve cover gaskets that came with my 87 Mustang were cork with metal inserts at the bolt holes to prevent them from being crushed. After a few years, the cork shrunk enough to actually leave the gap between the valve covers and the head. So oil was liberally leaking out, making big messes. I now have FelPro perma-dry metal-reinforced rubber gaskets that hopefully will last longer.

I just buttoned up my car after replacing the oil pan gaskets (also Fel-Pro perma-dry) and rear main seal. Took it for a short drive, and will check tomorrow for leaks. More on my other thread.