Just made an observation. Everyone of my gasoline-power cars's exhaust tip invariably gets partially covered with thin black residue. I have my A3 TDI about 3 months now. I've noticed that the chrome exhaust tip does *NOT* have any black residue on it, even though I have not been cleaning and wiping it when I wash my car. I'm guessing that it must be an evidence of how clean the diesel is.