HI T2,There is plenty of threads on this subject already, Anyway I had a simular experence a couple of years ago.I replaced the oil in both drive motors with the bobcat synthitic oil .one motor was full the other had a small amount of black smelly goo. 20 hrs later without warning(leaks, noise, power loss) the drive motor hub calapsed destroying the entire drive motor,10k later i was told that the bearings in the hub are under that extreme pressure it cooks the bearing/oil thus regular oil change.If i had replaced seals and brearing it would have been 1/4 of the price.As for oil my sugestion is a high grade synthetic gear oil not engine oil.