I have ran a multi-grade transmission fluid in the transfer cases of my last two trucks and never had any issues. I can't see where mercon sp would hurt anything though.
Less friction.... less wear.... all the better....
I've asked this question in several places and the answers are all over the board, from "he!! no don't use it" to "it's a much better product than Mercon V so,....Yeah, use it!", to everything in between.