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.

 

I think the heavier oil would be better for the heavy load you tow. Amsoil makes a 75w-140 for about $8.00/qt. I think I've seen Redline 75w-140 in Summit catalog. If you can't find either of these then I would put the synthetic 75w-90 in before I would get the 75w-140 petroleum. I don't know about the Mobil 1 but the Amsoil 75w-90 spec sheet shows it to hold up to high temps like a heavier petroleum and excellent under the 4-ball wear test which measures the load capability.
You might hear a lot of disagreement over which brand is best but I think if you stick with any synthetic you will be better off.

 

Where do I get this 75/140 oil? None of the auto parts places like Autozone have it.

 

75/140 from Amsoil costs as much as Motorcraft. Order it and it will be at your door 2 days later. Do a search using the oil weights. Ford used 75/140 so it would not be easily contaminated by water condensation and would not have a tendency to leak out the outer axle seals when hot. If you're still under warranty use 140 so there's no complaints later. However, many have used 75/90 with no problems, and it's factory fill in the 4x4 front end.