Sorry, I'm a little late to the party, as usual.
I actually used an ATF fluid in the primary, on reccomendation from a friend. I really didn't notice any difference in operation, but never felt comfortable having it in there. It bothered me so much I took it out after a couple thousand miles. I wasn't really worried about the clutch, but more concerned with the primary chain and sprockets.
On the other hand, I do use a synthetic in the primary, Mobil 1 Racing 4T 10W-40. This oil is made for wet clutch operation, and it seems to perform well. I think it will also provide a little better protection for the chain and sprockets. Maybe I'm just a little paranoid having had a primary chain break on me a few years ago. Ok, quite a few years ago.