[slotloco]

I used to "fluid dip" my motors. I got lazy and stopped.
I can't even be bothered flattening chassis these days - if it looks about straight and 4 wheels touch the track....

But the technique I used, was water with a few drops of dishwashing liquid in it - that sequesters the dirt and holds it in suspension, and if you have made a motor smoke without actually burning out the windings with oil on the comm/brushes, this also fixes that quite nicely.
3 volts for 10 to 15 minutes seemed to be about right for common RTR motors.
After the soapy wash, I would drop the motor into a container with methylated spirits in it, simply to displace any residual water, spin the armature by hand a few times, then stick it on a paper towel to dry off from the meths.

Some motors saw a clear, measurable improvement in performance, some hardly any.