And maybe another reason to use thinner oils:
Cavitation, a seldom discussed but often significant cause of wear and tear:
As stated by others here who are well read: Cavitation in journal bearings is more prone with 1) higher viscosity oil, 2) higher bearing speed, and 3) lower oil supply pressure. And too low a supply pressure means that the bearing is starving from inflow being less than outflow. Supply pressure is insignificant compared to hydrodynamic pressures and cannot approach them. And:
Effect of Viscosity and Speed on Oil Cavitation Development in a Single Piston-Ring Lubricant Assembly
by Jamshid M. Nouri. Cavitation occurs on rings too. This paper reveals less cavitation on rings of pistons with decreasing oil viscosity.
Influence of lubricating oil on cavitation erosion damage of engine bearing materials. I.J. Vercon and A. Rat. One of the main conclusions of this work was that with higher oil viscosity, at 80C, the erosion intensity increased nearly linearly.
The effect of viscosity on the cavitation characteristics of high speed sleeve bearing. Wang et al. Lower viscosity lubricants performed better than higher viscosity oils.
Ali