i was suprised when it was time to change the gear lube in my ford f250 4wd,it called for 85w140 in the rear axle,and the gears are large for a pick up truck.

I think its a temp thing. If the operating temp exceeds a certain number, the thicker oil comes into play. I don't think its as much as a strength thing as it is temps. Im sure ford had a valid reason to change the spec on that diff, as it used to be 80w90 I believe in the Dana 80, if that's what you have in yours?

My trans temp in my semi on a hot day fully loaded runs about 160-180*. I think if the gearcase temps start getting in the 220+ range the 140W oil works better.

I wonder if I should add a temp gauge on my dually rear end and see how hot it gets towing the boat. That would be cool, and a good way to determine what oil to run in it. Be nice to add one for the drives on the boat too! Of course synthetics play a role too!

that's my thinking...never said thicker oils were miraculous;they are able to whitstand higher temperatures without getting too thin
And if zero or five weights are so good in a nascar engine,who's going to be the first one to use it in his boat engine??
not being sarcastic...just curious,as im always willing to learn something

Thicker oil actually increases temperature.

Bravo Shop 75-90w oil will protect up to 400 deg. Talk to the folks that have used it and have seen unbelievable results not only in gear life but bearing life as well, they still look like new after being used for a season. We have been testing a 50-60w also in outdrives with the same results as our 75w-90.
We have temp senders for outdrives and I have adapter them to my dually just for the same reason Mild Thunder stated above and I now run Bravo Shop 50-60w oil in rear axle and in the manual transmission of my 04 Chevy.

Tried to make that clear in my original post. Jest experimenting to see if I can make my LH unit live w/o having to spend $1500. on a new housing.
Thanks for all the help guys. Keep it coming.

Now, this is a cool debate typically oil with wide viscosity ranges are used where tolerances change alot. NASCAR, and other racing organizations typically use low weight oil because it flows easier and creates less resistance to pump (free HP). Most "real" race engines are not loaded fully until brought up to operating temp to ensure the clearances are at a predictable size. A good oil, will hold the film size (based on the viscosity) at the temps they are designed for.

In my opinion, drive tolerances are wide enough to support the 140W (in multi-viscosity design only) number. But not enough to make up for being compromised due to excessive load.

agree that!

Do a Google search on REM polishing. I know some people in the race car world are having very positive results, with transmission and rearend gears.

Shhhhhhh,,was soppose to be top secret!..