About thinner oil...other things equal, thinner oil has less film strength. If the film strength is inadequate the hydrodynamic wedge of oil breaks down, there is metal to metal contact, and the bearing wipes. The engine designer balances the oil viscosity at the temperature inside the bearings, the surface area of the bearing (more area spreads the load and reduces pressure), bearing diameter for higher peripheral surface speed, etc.
The misdirected ramblings of the cosmetic plastic surgeon, Dr. Haas, never mentions this critical hydrodynamic lubrication. Hydrodynamic lubrication takes place even in oil bath bearings where there is no oil pump and no flow. The oil pump and flow is necessary in heavily loaded bearings so the flowing oil can carry heat away.