What probably happened is this: Your original setup pumping a higher viscosity oil needed less oil to the bearings because it wouldn't flow out of the bearings as fast; this caused your oil pump pressure relief valve to be operating most of the time, dumping oil back into the pan. Lowering the viscosity meant more oil could flow out of the bearings, less or no oil came out of the pressure relief.
Oil coming out of the pump is warmer than the oil going in, so bypassing some of that warmer oil into the pan raised the temperature. Stopping the bypass flow eliminated that source of heat.

R.