I think it is but it depends how much deep water you go through (deep water enough to flood the axles etc. requires a fluid change which gets expensive with synthetic).
I use Mobil 1 but I've used other brands too like Shell. I think the truck rolls easier with the synthetic gear oil.
Anything dino juice can do, synthetic can do better...Also dino juice can contain sulfates and other corrosive materials, where as synthetic's are designed from the ground up with their base stocks having no jagged molecules...http://www.trustmymechanic.com/motor-oil-bible.pdf