Difference between 75-90 and 90-140

[Goldenstate]

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?

[Mista Bone]

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

[Miller88]

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.

[nemo]

Fords recommendation for

99 Ford F-150

Rear Axle Lubricant
Motorcraft Synthetic SAE 75W140

Front axle (4X4)
Motorcraft SAE 75W90

[tpi]

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.