My only guess would be that Amsoil 10W40 that I'm currently running has some additive that gets activated at higher temperatures...
Shouldn't be anything in the Amzoil that's not in most other engine oils. Engine oils generally DON'T have any special additives for the tranny, as these tend to cause problems in other parts of the engine. But the sliding components in the tranny don't really need anything special to get them to slide.
It's normally the "dogs" on the gears that wear out, due to rounding off of the edges that engage to the next gear. This CAN be assisted with active EP additives, which ARE found in gear oils, but they can't be used in our combined sump engines.
Sorry to put the thread off track.
140 F is not a serious problem for the engine oil, but it is causing a decrease in viscosity. In my experience, the Bandit engines tend to start to run erratically, surging, etc after a long time at idle with minimal air flow. Try to avoid low revs/high torque under these conditions, until you get some airflow going again.
Otherwise, find a different road to ride on, as this sounds terrible for the "rider".