[jaydee]

The noticeable noise of the pump should be in the motor not the impeller or bearing. May be quieter because the oil is slowing the pump down because of the added viscosity. That also makes the pump work harder and probably shortens it's life.

Also using oil may reduce heat transfer as it may increase the boundary layer making the heat path from the source to the water and from the water to the radiator further and harder to travel. That and it will lower flow rate making the system less efficent.

Better way to lower pump noise is us a good DC pump and lower the voltage.