[iflyjetzzz]

Originally Posted by Glim

Pick up an oil filter, they come with the o ring for the cap, I bet during the last change they pinched the caps o ring when they replaced the filter.

Use a strap wrench and DIY:

http://www.e90post.com/forums/showthread.php?t=4768

That's another possibility. I have one of my old O-rings that I saved, just in case I damage a new on while putting it on.

OP, the O-ring is black; sort of like a rubber band except that it's less 'stretchable'. I've attached a picture of the oil filter cap with the O-ring on it. The top arrow points to the O-ring in question.

Edit: This isn't an E-90 cap because the E-90 only has one small O-ring at the bottom. But the photo should give you a great idea of what we're talking about. Note that the O-ring is not flush with the cap lip; it sits in a groove ~.5mm below the lip.