Dirty Driving Shoe

Took my new (to me) '86 300E on a road trip last week. Drove 500 miles, averaged 24.5 mpg at approx 70-75 mph. Was very pleasantly surprised. Also learned that I burned a quart of oil during the cruise when the oil light came on during the return trip. So now I know I need new valve seals.

But my reason for this post is to ask if anyone knows what might be blowing on my right foot. I am finding a black coating on the upper right side of my white sneaker after every ride in the car. It doesn't appear to be oil because it will wipe off with water and scrubbing without leaving a stain. Could it be the interior of a heating vent pipe coming apart? Part of the problem might be from resting my foot against the side, but I do not see an accumulation on the the widest part of my shoe (the Ball of the foot), just the area above it to the toe.

Thanks,
--Craig