gear oil or synthetic????

All of it used to have a distinctive smell, however now I find some oils labeled synthetic are pumped right out of the ground just blended with other base stock pumped out of the ground. You can even find some info about that on this site using the search block, top right.
I have sever seen 85-140 synthetic so I assume it is not.
I never saw any immediate problems mixing gear oil unless one was mineral oil.
If you drain it all out and fill it back up that is not considered mixing it, but what about the hubs?
Why do you want to go to 85w-140?

 

Doubtful the rears or trans come from factory with anything other than synthetic.

 

It really doesn't matter if you run one or the other. No-one puts dino in diffs or trans anymore. Manufacturers probably haven't for more than 20 years. Synthetic is the way to go. Lower long term cost and much longer lasting.

 

The company I worked for in the early 80's ran Rockwell rears with 75w-90 and Fuller trans with 50w. Said Rockwell would warranty them for 600,000 miles, except abuse, but they had to run synthetic oil. If you are running OTR, drain out synthetic and fill with 85w-140 the diff. operating temp. goes from about 175 to 240.
They only changed it due to water contamination or failure but they never kept a truck to a million.

 

Not sure why you would want to use such a heavy gear lube in the diffs. The factory fill is 75w90 synthetic. I use the same 75w90 that was the factory fill also in the Conmet hubs on my steer axle. I use the same 75w90 in my pickup and my Cadillac also. Can't speak to every make, but Eaton and Meritor both extend the factory warranty of 500,000 to 750,000 miles if owner uses synthetic in the diffs and trans.

 

Best bet is to run what the OEM recommends for your ambient temperature and work conditions. Their engineers are (usually) smarter than most of us.