Chris wrote:

How would a motor handle the increased turning resistance
caused by the oil filling? Do you think a trolling
motor would survive being oil filled?

If you want to reduce the internal friction to the minimum you "pot" the
armature in epoxy and then machine the excess off to leave a smooth
surface in place of the less than hydrodynamic surface of the original.
Or just leave it alone and not lose enough power to worry about.

Rick