There are several different things being talked about here and hard to keep up.
ATF+3 was discontinued in 05 and replaced with +4. If you have some old fluid still to use in an older than 05 vehicle, you'll be fine in both and auto tranny and tcase. The main difference between the +3 and +4 is the +4 went to using group 3 basestocks(synthetic??) as opposed to mineral based.
Dexron is GM's rating on fluids and really shouldn't matter for this discussion.

The NV4500 does need a GL-4, not GL-5 rated 75/90 gear lube.

 

For the most part. Synthetics control heat better.(usually) That and the last time I priced it, The Amsoil synthetic fluid was cheaper than the Mopar fluid by a good margin. IIRC The Mopar fluid is The Castrol Syntorque anyways.

 

I went ahead and bought some amsoil just because it was cheaper. Whats the point of buying fluid that is higher and its pretty much the same thing

 

Some transmissions shift different with different fluid. My nv5600 actually shifted worse with the amsoil fluid than it did with the mopar fluid.