Pepsi! i guess it is a preference thing. The kind i use is the full black rubber(no metal) ones from NAPA, they are really pricy. But you get what you pay for. And to add to the confusing i use no silicone.
If you use non hardening Permatex on the valve cover and a thin film of grease on the head side,it will never stick to the head,or leak! You can remove it very easily and use it over. I can't emphasize enough not to overtighten cork! It will swell on it's own,just by absorbing oil. It will take heating and cooling better than rubber. Some of you may remember back in the 50s-60s,when you had two hold down studs with nuts on a valve cover! They didn't leak for years! Now we have twenty bolts or screws on a valve cover and oil leaks between them! Sorry we have to waste a few trees,but IMHO cork has it all over rubber!
