That sucks so bad. I accidentally did that one day, luckily I heard the oil cap fall off and I pulled over to check what it was. Went back and found my oil cap laying in the middle of the road. Yeah, definately get it cleaned up. Anything rubber can degrade if it gets coated and stays coated. If you do have a rotary this won't apply but if you have a piston engine and you have a timing belt you may want to check and make sure that it didn't compromise it any. Petroleum products own rubber products, that's why you can't use vaseline with condoms! It makes them look like swiss cheese on a microscopic level!