I've been noticing that on my BBS wheels of my 2007 STI, black grease-like material build up after a few days of driving. Can someone explain this "phenomenon", since I never get this black material on my 1999 Chevrolet Malibu's wheels.
This picture was taken right after I drove the car for 3 minutes. Prior to that, I just finished washing and cleaning the car (including the wheels):
For a closer look, here is the black material stuck on my hand: