OK i have a 99 stang with 17 inch cobra rims. i keep getting this black stuff on them and the back ones clean up fine but the front ones dont (cuz of the brakes?). i presume that it is oil but i could be wrong. what can i do to stop this from happening? is it something that is a maintenance problem or is it normal and i just have to live with it? the front ones dont clean up...would sand paper get it off or would it scratch my rims? thanks for any help!