My apologies, I skipped off to other distractions without following up on this thread....

So, my interpretation on the original post was the distinction between cork vs. rubber gaskets as opposed to cork and rubber gasket material.

For as long as I can remember, (not long apparently), valve cover gasket material was a combination of cork, rubber and some type of binder. Never had a problem with that material, but as Nick points out, the solid rubber examples tend to leak if any oil gets between the gasket and sealing surfaces.

Cork's magic is it's ability to absorb oil, swell, to fill gaps evenly on un-even surfaces. I'd have to lean to the cork and rubber version over the solid rubber. Haven't tried Nick's neoprene, might give that a turn one day.

BG

I never had much luck with cork gaskets unless I glued them on. Then You had to chisel them off or wirebrush them. The best gasket now is the preformed neoprene with metal in between. Thicker and they will not leak. I first used them on an LT1 engine. I used to use the rubber but they would tend to slip. Modern day materials. NICK L72