Borg Warner overdrives are notorious for leaking the overdrive oil into the differential. The reason is all the BW's are cottage industry modification and the rear seal used is a grease seal, not an oil seal and the oil gets past it. It is prudent to check the oil level in the differential every few hundred miles to see if it is manufacturing oil. If it is, three things are likely happen.
1. You will not become a rich oil baron.
2.Oil will overflow the differential out to the brakes.
3. However, before that happens the overdrive will starve for oil and fail.
Tom Endy