crocee,
Sometimes people over oil the motors. Then the oil seeps out and spreads everywhere. When it gets to the commutator and brushes you'll have trouble.
I think another thing that happens is (at least in the case of my Edison) is I get a dry motor, oil it up good then set it aside while I'm working on the machine. In the Edison's case it took me nearly two years to learn enough to fix the thing. So all that time the well oiled motor was just sitting there with the oil (no doubt I put too much in) spreading to the brushes and what not. Yuck.
Joe