Paul
The Borg-Warner T5 transmission in my truck uses ATF for lube. The first transmission I used had gear oil in it, which was it's demise. The T5 has rather small oil passages to lube the gears, and under cold temps, gear oil doesn't flow well as you know. Hence, lack of lube made the tranny shift like crap, though it never actually failed totally. So the ATF is less viscious, and lubes the T5 better. Besides, if it can take the torture of an automatic transmission, which still has meshing gears and bearings, then a manual should be easy for the oil to handle.
Also, as an aside note, the T5 had a designed max rating of 250 hp, so in theory, with proper maintainance, behind the C223, it should last for just under a million years. <img src="/forums/images/graemlins/notooth.gif" alt="" />
I have no data to prove it, but I believe ATF has all the anti-shearing properties we would need in our applications. It would have less drag and therefore less HP loss. Modern oils are much better then they were, even than 20 years ago.

My Isuzu manual calls for 2.7 pints of SAE #30 motor oil in the gearbox.