Madmaxx you may be right about the effeciency change being negligable. I never worked with auto
oil pumps. My experiance is with very large, high pressure pumps. I have seen them drop as low as 40%. It all depends on the clearances and viscosity. Smaller pumps do have less losses.
I think I made the wrong arguement. When the
oil pressure in an engine reaches the limit on the prv built into the pump, the prv lifts and
oil flows back into the pan or the suction side of the pump rather than through the engine. If the prv is set at 60 psi, then when the resistence to flow through the engine reaches 60 psi the flow is diverted through the prv (ususally internal to the pump). Therefore increasing viscosity of the oil will reduce flow to the bearings if the pressue reaches the prv setting. I have seen people shim the prv spring to get 100 psi on an engine that would normally only run 60 psi cold. I'm pretty sure that most engines lift the prv when cold even with light oils, as the oil pressure stays steady when you rev the engine.