They wash away, I promise. On my little benz's it's easy to get the cap on wrong. The fuel tank is up in the trunk, higher than most vehicles, and when the tank is more than 3/4 full and the cap is not on correctly it will dump gobs of fuel out on every corner. I left a large wide streak from my barn all the way past the corner at my house, couple hundred yards long. It was 90% WMO. can't even tell it happened 9 months later.