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