Oil it. All axle bushings, gear posts, and a DOT on the armature shaft ends at the bearing /shaft. (oil wont kill the windings/armature/etc, but it is messy.
A little grease goes on the gear teeth.
Repeat this often/every few hours. Black oil= wear. Flush it away (wd-40/crc electrical cleaner/etc) re-oil. (wd,-40 is not a "real" oil..Common .Mobil 1 etc will do)
Look for threads on motor tune-ups for an in depth cleaning guide. ( "universal" type threads/suggestions vs #specific etc.)