There are a couple of different scenarios at play here Greg, each with different circumstances, needs and results. In regards to strictly gears...I agree that new ring and pinion gears need dino oil used with them until they are broken in. Once broken in though, I see no reason why synthetic gear oil can't be used on ring and pinion gears. My ring and pinion gears were broken in on dino oil but have had synthetic run on them for the last couple of years...and they have seen a ton of abuse and still look perfect.
Now, differentials...are another circumstance. Gear type lockers...I'm not sure what's best to use with them and when as I've never owned one.
Clutch type posi's though...from one horses mouth that builds and races a lot of these...dino or synthetic gear oil is fine either way...as long as a friction modifier is used to help the clutches slip. I can't see why being "too slippery" would be a deterrent at least when breaking in a new clutch type posi, especially after dealing with this the last few days...
Now I'm not an expert, but I have just listened to a LOT of them...with lots of different thoughts on this.
I just wanted to post this to show that not all diffs are apples to apples, different scenarios are at play here.