The manual calls for 75-140 in the rear, but I doubt that you will have issues if you don't push the truck too hard. The extra viscosity helps with wear under high stress.


Just for reference, this is what I did in mine:

 

^^^ X2 ^^^ but, I used mobile 1 syn

 

I did the same in mine. Only had the lucas in for 10,000 miles before I had to drain it to change the disk brake mount though.
I have Supertech 75w-140 and the XL-3 in it now.

 

How is the ST fluid working for you?
Syn or conventional?

 

It is synthetic (as far as I know, all 75w-140 is syn).
Only been about 5000 miles with it though, but I have never had an issue with any ST fluid.
The ST with the XL-3 has no clutch chatter or noise (but do have a pinion seal leak, but had that before).

I used Valvoline 75w-140 with the XL-3 in my brother in laws '04 and it works fine as well.

 

The same axles in the 2016 owner's manuals state 75w85 fluid to be used. That's even on a max-tow 9.75 differential.

 

Different years, different materials are likely in use. never use a different model year's manual regardless of how similar the parts appear. You don't know what materials are used in seals, clutch packs, etc.

 

That also tells you to use mercon lv and not v...as stated, different materials, clearances, clutches, etc.

 

If we really really want to get technical, Ford doesn't even recommend changing the diff fluid. They claim it to be one of them new fangled "lifetime" fluids lol. But here we are

 

Yes, but "lifetime" is a relative term.
The lifetime of the warranty period? Lifetime of the vehicle. Lifetime of the owner.

Vehicle manufacturers care about getting the vehicle to 100,000 miles (heck, some say just past the warranty period), after that, all bets are off.