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If you mean valve cover gaskets, I've never had good results with rubber. Since I have solid lifters, R&R of the valve covers is something I've done a lot.
+1 on rubber if you are pulling them off and on, but in my experience tend to squeeze out if not torqued right. If they are staying on, I'd go with the cork held in place with some yellow adhesive....
The Fel Pro rubber/silicone with the steel laminate core and reinforced bolt holes, like James said. You torque it down correctly one time and it's fine, it doesn't swell and shrink and get hard like cork does. I've used both and the steel cored rubber is 10x better, you just have to make sure the mating surfaces are clean and straight.
Re-inforced rubbed is the only way to fly. They do make a black black viton type rubber though that works well with stock thin railed valve covers. You cannot beat steel cored rubber gaskets on a good set of thick valve covers. Off and on. Had the same pair for over 5 years.
I googled fel pro gaskets and was directed to JEGS.com and couldn't find any for a 6 cylinder. Only for performance engines. Well, excuse me! Any idea who might have the silicone coated steel valve cover gasket that is reinforced at the fastener holes for a 6 banger? NAPA has a cork reinforced with rubber gasket, and the plain cork.