My two cents worth.
Heavier weight oils are fine for engines with increased clearances, but for stock clearances a reduced viscosity
oil is required to insure
oil flow.
Oil flow is what ultimately keeps the bearings cool and if the oil flow is inhibited by a higher viscosity, then the bearing gets hot and ultimately fails.
Overhead cam engines require very low viscosity oil mostly due to the extreme lengths the oil must travel to get to the heads and supply an adequate flow of oil, especially when the oil is cold.