Just what exactly is this? Something special or ordinary gear oil by just another name?

I'm wanting to change the front differential fluid on a '95 F250 4x4. Front axle is a Dana 50 TTB. Owner's manual says "4x4 gear oil".

I get wild search results without any solid answers for my specific application.

 

Never heard it called that before. Personnaly I'd dump in 85w140 and call it a day.

 

Here is a link to all of the different kinds of transmission fluids Ford uses. At the bottom, under the type F tranny fluid picture is a link you can use to look up your vehicles Ford fluids by VIN #, to see if there has been an update on the recipe.
Once you have that, you can cross reference other manufacturers p/n, if you can't find the Ford product you want, so you get a recipe thats friendly with your trasnsfer case metals, seals & gasket material. Motorcraft® Transmission Fluids ? The Official Ford Parts Site | FordParts.com

 

That didn't work for me. It's as vague as the owner's manual.

Why 85w140? I am going to use 75w90 on the rear, probably will do the same on the front.

 

Transfer case fluid is what you want I believe. There is more info down in the 6 & 7 notes on this page in the axle & tranny lube charts. GL-5 axle lube shouldn't go in a transfer case if it isn't specified, as the sulphur phos ad pack might not be metal, seal, or gasket friendly. The specified viscosity range in our manual is there for a reason, imo best for us to adhear to it. If you have a limited slip differential, it also might call for the specified friction modifier xl what ever is specified for you to also add to your differential lube. https://www.fcsdchemicalsandlubrican...n/quickref.asp

 

75-90 is perfect for the front axle. 140 is going to get VERY thick when cold. The front axle has no clutches so it doesn't matter if the oil has any specific additive packages or not. Any 75-90 GL5 will be fine. The front axle is as low tech as it gets.

 

Most front axles I have looked up call for 80w90 so 75w90 would be just fine. I put 75w90 full synthetic in my front diff as an upgrade. I like to use 75w140 in the rear diffs even if they call for 80w90. I also prefer to use full synthetic gear lubes. Just personal preferences but I remember an owners manual for a GM truck I owned that spec'd full synthetic as lifetime fluid(unless submerged in water) or change conventional 80w90 every 30k miles.