in winter time 140 is to thick for my 2 troy bilt horses I use 85w 140 so as not to burn belts up. MTD.com search there for parts and link below lowes can order parts for you and a lot small engine repair shops like old western auto store that has survide.

Here is a link that might be useful: tiller parts

You need to make sure the gear oil is GL-4 (not GL-5). GL-5 gear oils usually have a sulfur limited slip additive and it'll eat up the brass that's in the Troy Bilt trannies.

Some synthetic gear oils use a different additive that's safe for copper and brass.

The one I use is Royal Purple Max Gear. It comes in 85W-140 and is safe for GL-4 applications.

Standard GL-4 gear oil isn't easy to find. Your best bet is farm stores.

P.S. Here's a pretty good source of old Troy Bilt parts:

http://zacm.com/brand/troybilt.html