This is always an interesting topic.
But there are so many other things that would affect the performance of a motor. My experience is mostly with can motors. We are not building speed machines here nor dynoing them for performance. Although, I’m sure we would all appreciate a strong motor with a nice balance of speed and torque but which keeps amps at a reasonable level. I would opt for a motor that is efficient.
As far as oil on the brushes… There is such a thing as oil impregnated brushes. A rather simple process as outlined here http://www.freepatentsonline.com/5798178.html
I would say, maintain the motor to the best of your ability and give it a test run on the track. [:)]