Some years back, I changed the gasket. I think I got it at Autohaus AZ or some other place, Authentic Classics has the cork style for about $7.
The ones on eBay A1404710079 are crazy priced. $25 and more for a piece of rubber, totally ridiculous. Some were $68!
Mine is a blue rubber. The only issue I have is that it is a bit thicker than the cork and thus you really have to be careful (particularly with those cost like gold chrome gas caps!) to ensure it is secure. A little lubricant on the seal makes it easier. It's a bit of a bear to secure. Sometimes I need a glove to do it.
I'd have to agree that silicone would be an improvement. But I'm not going to pay $25-$68 for such a thing. Be your own judge.
As for the spillage, well--Len has it right. Don't top off. The alcohol soaked rag is a good cleanup scheme.