I'll be doing front/rear diffs on 15 F150 4X 5.0 not too long from now. Any issue using Valvoline Synthetic 75/90 in both? I don't tow anything but bikes, canoes, and camping gear (family trip truck not a DD) so a 140wt in the rear isn't really necessary for my application....at least I don't see it. If I'm wrong, please take me to school. I'm not positive what the spec is for this but I'm guessing GL-5?? The Valvoline meets it. Missing anything here?

 

if you use to thin of a gear oil you run the risk of destroying your bearings

FWIW both diffs call for 75-85

 

85 is more of a pure gear oil with no additives (supposed better MPG) and the base oil is thicker, while 90's base oil is heavier and has additives that assist the oil to protect the gear spikes from wear off and catching rust. It minimizes corrosion due to the constant vibration and never-ending friction. It also reduces chattering in limited-slip differentials. No difference with running either of the oils as long as you use synthetic oil

 

unless you somehow have a limited slip front and rear diff (im 99.9% sure you dont). That is the wrong gear oil.

 

Go ahead and use that oil with confidence. Nothing wrong with that choice at all. Ford calls for a 75w85 to eek out as much fuel economy as possible.

that 75w90 is just slightly thicker and will cause absolutely no problems

 

Pretty much all off the shelf oils have a limited slip additive. No harm for a non limited slip diff.

The Ford spec has changed many times over the years, 75W-140, 75W-85 Motorcraft... I agree with others, the lower viscosity Ford fluid may partly have been chosen for fuel economy.

It's not what is in the owner's manual but I think the 75W-90 synthetic is a good choice and this Valvoline in the flex fill bag is what I used in my front/rear recently. And slightly thicker may have better protection (thinking about members with 4A front diffs that failed).

https://www.werkenmetmerken.nl/docs/...GL-4_75W90.pdf
https://www.fcsdchemicalsandlubrican...0Lubricant.pdf