75w-140 in a Dana 30
 

I’m about to change the fluid in my Dana 30. I have some 75w-140 left. Can I use it in my 30 or should I use 75w-90?

If memory is good, 75x-140 is what the manual calls for when Towing.. in the Rear.. Go with what the owners' manual specifies; the engineers have a reason for that specification.

I tend to agree. I'd use 75W90 in that D30 but you are fine to use the 75W140 in the rear D44 in my opinion, even if not towing and since you have it, and so it doesn't go to waste.

The manual say 75-90 as the front diff usually sees less severe operation compared to the rear. Both have the same low temperature viscosity. The 140 stays thicker as the temperature rises that is why they recommend it for towing.

Yes you can use it.