I'm switching this week to a high temperature synthetic.
I installed a cyl temp gauge and the first time on the road in warm weather the head hit 370 degrees. Some oils start to break down at just over 400...
The meter is reading the very hottest part of the engine (spark plug) and who knows how accurate it truly is, but it confirms something I'd heard before... these big singles run hot.
Therefore, synthetic for me, starting immediately.