If you have an engine with an
oil cooler and several feet of hose and could take
oil pressure readings from
oil exiting the block , through the cooler, through the filter, back into the engine, up to the valve train, you will find that the pressure will continue to drop. FLOW RATE is affected.
Correct pressure will drop as you move down the circuit because the resistance in the system is dropping. Total flowrate from the oil pump will stay the same.
The oil pump has to push all that oil through the system which has RESISTANCE. To take it a step further heavy cold oil will have less flow and a greater pressure drop through the system at a SET pressure.
Cold oil will not flow any less than warm oil assuming your oil pump relief valve does not lift, you will have more oil pressure not more flowrate. There is less than 1% slippage in most positive displacment pumps.
What you have to remember is the pump has the relief valve on it (that is your pressure regulator). It pushes oil so when it hits a preset pressure it will not go any higher and will not overcome the resistance. Pressure is highest at the pump.
I agree, the relief valve lifts and recirculates to pump suction.