I have never had a problem with the copper ones, the only reason I have changed it was so I could re build the engine because of an odd sound (turn out it was the timing chain)
So I fitted a steel gasket, I have only done a few miles since fitting but at the end of every little drive I find the oil cap to be white. I think really this is a sociological thing

I have looked at the dip stick, that's not white on the bottom, but I can't tell if the oil on the bottom is water or just runny oil :/
Please could someone help!
Thanks
David