First, I think it's safe to say that most of us are using ATF in the primary case, though, it's not what was originally specified.
As for the Mobil Racing 4T, I think Mobil has put out some confusing information about this. If you look at the 10W-40 version, their advertising says it's specifically designed for wet clutches, but the 15w-50 does not say this. In fact, I found this post on another forum:
http://www.triumphrat.net/sprint-forum/99074-reply-from-mobil-on-their-15w-50-a.html . In this link, Mobil is suggesting against using the 15w-50 for wet clutches and goes on to suggest using their V-twin oil, instead.
Also, for 10w-40, you can find a PPM number for zinc (1200), whereas I was unable to find a number for the 15w-50. By my understanding, with flat(tish) tappets like ours, the more zinc the better. I use Valvoline 20w50 VR1 Racing Oil with 1300 PPM zinc. As it turns to out, the V-Twin oil has about 1800 PPM zinc, so I'm going to look into that, but continue to use ATF in the primary.
Richard L.