I use 75w140 Royal Purple(because I found it on sale) in my T/C and front and rear diff.
It has made everything roll smooth no wine. I have not noticed my mpg decrease b/c of the heavier weight oil. From my research the DII requires synthetic fluids. Even the motor oil from the factory is a semi Syn. Its up to the driver I guess. I like Mobil1 some like Rotella

 

It depends on which fluid you're talking about.
The transfer case and diffs are essentially the same as used on the D1 which came filled with dino lube.
The diffs are exactly the same design as the diffs used in in my '62 Series IIA. So obviously they don't "require" synthetic, of even semi-synthetic, in that application.

 

I see you point AntiChrist,
After 100k I think its best to prevent premature wear....to run Synthetics. It seems drain intervals can be longer with synthetics due to the great lubricity. If thats a real word. Anymore I know there are only really two companies that are full synthetics. Amsoil and Redline. I think most other manufactures put in additives to increase the wear time and tolerate dirt contamination.

 

I just filled my TC with royal purple 75/90, I used to run the Vavoline HP 75/90 and the whine was pretty loud. With the RP the whine is all but gone and the drive seems alot smoother (could just be me wanting it to be better at $20/qt). I used to believe that there was no difference between syn and regular oils, but owning this vehicle for about a year i find that the truck seems to run alot better with synthetics.

Just my 2cents...and I have no technical details to support it but from now on it will be all synthetics for me...

 

Maybe. On the other hand my diffs and transfer case looked fine after 265,000 miles of straight dino. But I also change it frequently as I off-road a fair bit in mud and water. That's the main reason I run dino, so it's not so expensive to keep it changed so it's clean.

 

Exactly, thats the same reason I run dino lube, its cheap and I change it once a year because I also wade alot.
No way in hell I'm going to spend $100 on gear lube just to drain it out after wading because I had a leak I didnt know about.
And nothing requires full syn.
All lubes are rated, as long as you meet the required rating for your application you are fine to use whatever you want.
But a full syn will last longer.

 

wait...no Spike's famous wading video? I'm disappointed...

 

LOL!!
I almost posted it, almost...

 

Not a video, but a friend doing type of wading I'm not a stranger to.

 

That's more like wallowing in a mud hole.
I think wading implies some movement