Been reading all this oil viscosity thread with interest.

        Lowering the viscosity should raise the Reynold's number.  The increase in the Reynold's number will directly increase the heat transfer. It should be a very strong effect.

        Lower viscosity should also decrease the pressure drop across the cooler which could possibly increase the flow. If the flow increases, this will also increase the Reynold's number. With a positive displacement gear pump, however, I think the flow change might be small, perhaps none.

        Awhile back, Tracy mentioned that higher input water temperature to the radiator would increase the efficiency of the radiator and the output temperature would not increase. This is not 100% correct. Indeed, the radiator efficiency will increase, but the output temperature still will go up a bit. The change in outlet temperature will be some fraction of the change in the inlet temperature.

Bill Dube'