Taken from here: https:/
I thought it was the opposite and synthetic oil is better for cooler running due to reduced friction?
Synthetic oils typically have quite different heat transfer characteristics to mineral oils. On modern water-cooled units this is not a problem, but on the air-cooled unit where a significant part of the overall engine cooling comes through oil cooling, the use of synthetic oils can cause problems. As the amount of heat dissipated by the oil is reduced, the engine runs hotter.