I was submerged in water. Needed to change my diff fluid. Guy at napa said use 75-140 synthetic. $16 a quart???? If this correct? 4.4 quarts in the rear and around 3 in the front would cost a fortune to change.

75-140 for the rear if you tow definitely, but I'd stick with good old dino oil; not the synthetic unless you have a limited slip. Syn has a tendency to absorb the water, not displace it like dino oil.

75-90 for the front; or FR & RR if you dont' tow much at all.

There is an on going debate about this. Search a little deeper for more info.

He meant pints not qts.

I put 75-90 syn Mobil 1 in my front and rear difs. I don't tow or go offroad.

I tow occasionally and go offroad. So Which diff oil is best?? I know you guy's know the answer to this!

75w-90 up front and 75w140 in the back. If you occasionally tow, then you might as well be set up for it.

If you sink your diffs often don't bother with synthetic - can get very expensive in a hurry. Otherwise yeah use 75w140 if you're beating on your Jeep or have lockers.

I put 80w-90 in my rear recently. Just the cheapo Napa stuff (still 8 bucks a quart, ugh!)

What's the big advantage 140 has over the 90?

Better film strength, shear strength, more stays between the gears when compressed, and better heat tolerance (doesn't break down as fast). Also in some climates people report better 'feel' since the synthetic oils are nowhere as thick when cold. Probably ups your fuel economy a little bit first thing in the morning in the winter.