An analysis of Mobil 1 V-Twin, 20w-50 showed a higher amount of moly than the 15w-50. So I've been using the V-Twin in the engine and just use Harley's stuff in the primary/clutch, and tranny.
I get it from Advance Auto Parts. Sometimes Wal Mart has it, I just picked up 3 quarts from Wal Mart a couple of weeks ago, it was all they had, last week they didn't have any.
I've used Mobile 1 in my vehicles and in my harleys since the mid/late 70's and NEVER had any problems - engine, transfer case, differentials, transmission, or primary drive.
Mobil one 15w50 in the engine, cheap ol' dino 10w40 in primary (mobil one is too slippery for clutch) and Mopar 75w140 synthetic gear lube in the trans (free for me at work)! 7500 miles of trouble free and quiet operation! (knocking on wood as I type). Good luck buddy!
I use Mobil 1 15W-50 in the motor, Mobil 10w-40 in the primary, which I may try a Mobil 1 10w-30 next time, which would give a bit more friction to the clutch. And in the transmission I use the Mobil 1 75/90 gear oil. Just my 2 cent.
I tried the mobil one in the primary and I swear it was slipping when I took off kinda hard from a stop. I was told by many that plain jane 10w40 is best! No need to waste money on fancy stuff in there.