[markjohnson]

I've been using the black GM ribbed type for quite awhile now. GM #14085759. In fact, I don't even consider anything else anymore. It's a no-brainer. They have a steel core and retain their shape. A lot of rubber valve cover gaskets are really floppy and flimsy and I've seen cases of them squeezing into the inside and rubbing against a couple of rockers on each side that happen to be very close to the top rail. Heck, I've even stacked them on top of each other to hide expensive valvetrains in my street race days and they always remained leak-free.