Engine Oil Pressure on SB Mopar?

My son's 318 takes a few seconds to get the oil pressure up to the middle of the scale on the factory OP gauge. I know the sensor is on the top of the engine and should take *some* time, but how long should it take? It starts coming up after it starts and continues slowly up. The OP gauge is the original factory gauge, could that be the problem? The engine has been rebuilt and it has a new oil pressure sending unit.

bwhitejr