Where did you get that info? If you'll look at the additives used in each there isn't enough difference in the two to make any difference at all in the efficiency or cooling of the engine. Mobil 1's position is that the 15w50 is well-suited for use in a Harley engine. If anything, it has more friction-modifiers because it is designed for use in engines that don't share their oil with the clutch pack. The V-Twin has reduced additives for MC's sharing the oil, as too much can cause clutch slippage. Less friction is good in an engine, not good for a clutch pack.
Mobil 1 article
I've seen three oil tests by MC mags and the M1 15w50 was an excellent performer in all of them. What makes it a standout is that it costs $22/5-qt. jug at Wal-Mart.