On a whim I decided to check the rear axle gearcase (I guess it could be called the differential, though that's not really what it does...) on my brother's Moto 4 350, as I'm sure it hasn't been checked since ownership. I was kind of surprised to find that there wasn't oil in the case, but some kind of a thick, foul smelling grease. I've Googled a bit and couldn't find a definite answer as to what is supposed to be in there, but given that it seems to have a drain plug I would assume it's supposed to be gear oil. If that's the case, would it be worth the effort to replace the grease with oil, and if so, how would I go about safely flushing out the grease? The grease smells bad, but it doesn't smell rancid (as if water got into it).

 

Thats not grease, it's ancient, solidified gear lube, decades of neglect have turned it into the mess you see now. Flush it out, spray out the inside with brake cleaner and leave it sit to let all the mess drain out, and fill it with a good 75-85 or 75-90 GL or GL-5 rated gear lube. Hopefully no real damage has been done, more than likely the bearings in there are not in the best of shape, get it all cleaned out and go from there.

 

Ooh, can't believe no one's ever changed it in all its years of operation. I'll have to flush it out when the weather becomes bearable.

Thanks a lot man.