F250 Heavy Duty differential oil?
I have 1989 F250HD 2wd. It has that fully floating axle (Dana 80 which takes about 4qt of fluid)
I want to change the differential oil, and I got Mobil 1 75w-90 synthetic gear oil. Now, however, I hear that it is best to use 75w-140 in heavy-duty applications, as it is OEM.
So which one is it?
Ideally, I wanted 75w-140 in synthetic but couldn't find it anywhere. Will 75w-90 M1 suffice or is it a mistake? I sometimes tow a signficant load of about 9,000 lbs. I have a feeling I should go back and exchange it, but my only other choice is dino 75w-140.
Also I know Ford sells their OEM stuff but it is pretty expensive per quart $20/qt, certainly more than M1, and I don't think it is synthetic.