Greg, My understanding about the zinc and phosphorus additives is they are for anti wear in high load conditions, Cam lobes, flat tappets, etc. The modern oils include polymer compounds to replace the zinc,etc. because the catalytic converters on modern stuff can't burn off the zinc and it fouls the catalyst. I believe it is the polymer compounds that fuck with the clutch operation.
That said, I have a 95 Triumph Sprint that has run the Mobil1 15/50 since it's first oil change with no issues.But it is my understanding the 15/50 does have higher ZDDP levels than the other Mobil 1 oils.