@MIX5 :
This kind of wear/pollution can under certain circumstances occur after less mileage than I mentioned.
Eg softer coal brushes wear quicker,and leave residue sooner.
Only way to find out,is to open your ventilator motor...????
I do understand,that when you have a newer model car,you are not likely going to experiment...
I also must stress,that I donīt recommend this reparation to none skilled car lovers...
Fr.gr.georgevan