Oil + additive viscosity

This isn't about film strength or shear. The viscosity or resistance to flow is impacting the flow through the oil system. By flow I mean volume in a given time, such as gallons per second. A high viscosity oil does not move through the passages as easily,and this creates higher pressures at the pump for a given rpm. When the pressure at the pump overcomes the relief valve, a portion of the volume is diverted back to the sump. These are two reasons why a higher viscosity does not mean the bearings and the top end will get sufficient volume. In fact it can be the opposite. The clearances and the oil's temperature are major factors in selecting a grade with the proper viscosity range.