Difference between 75-90 and 90-140
 

Has anyone had experience with what advantage one might gain in changing from a synthetic 90-140W oil in a differential to the thinner 75-90W synthetic product?

Was wondering if the change would be worth the expense and time involved?

Expense could be massive if you burn up the gears from heat from too thin of a fluid.

I drive my Jeep very infrequently, so my fuel economy data isn't that accurate. However, with 85w-140 it wouldn't creep in gear when cold. It also felt like the brakes were dragging unless I drove it until the fluid got warmed up.

The 80w-90 is a much better choice (for me) year round. It's also what is recommended in the owners manual. I was running it only because I had a bad front pinion bearing that was leaking. So I decided it would be a good idea to put it in both front and rear differentials.

I changed back to 80w-90 last summer.

What about a 75w-140? The 85w-140 was a "dino" oil.

Fords recommendation for

99 Ford F-150

Rear Axle Lubricant
Motorcraft Synthetic SAE 75W140

Front axle (4X4)
Motorcraft SAE 75W90

I had a '97 F150 which needed a differential rebuild once in the 70,000 miles I had it (using stock lube). I then bought a '99 F250 w/ 10.5 ring gear differential. I was commuting with it-all mild flat freeway- and swapped out the fluid to 75W90 thinking my situation was easy on it. I did tow about 5K miles in 110,000 miles. Well....that differential was screaming loud at 110,000 miles and needed complete overhaul. I had changed the fluid to Amsoil synthetic at about 20K so the lighter than spec fluid was in 90K.

I've owned a lot of vehicles and very rarely had differential problems. I will never be tempted to use lighter than spec gear lube in any Ford products again.