Ila,
If you just oiled the motor, yes that could be the problem. FW motors require grease, not oil. If you greased the motor and oiled everything else, just run the motor at max speed for a few minutes till it clears all oil out.
To clean the commutator (the part the brushes fit against) just gently remove the brushes and clean it off with a q-tip soaked with alcohol. Clean the brushes, springs, brush tubes and caps too. Then run it at max speed for a few minutes.
Hopefully that will clear it out.
Joe