The stock gearing is fine. Gear mesh is important. Do not put an 8000 mah Lipo pack in them and go and run them for a hour that will kill your motors quick. Driving style plays a part as well. Alot of short quick burst will cook brushes pretty quick. I also see that everyone is cleaning there motors. Be sure not to use brake cleaner as i have seen people do invest in some decent cleaner. Also when you oil your bushings you only need a drop on each one. Remember is oil attracts dirt and dirt is a killer on any electric component.
Not an expert but this is what works for my motor.