Just took a joyride in the duster

60 psi actually sounds a bit high, so no worries. I would think the drop is normal. As the oil heats, the pressure relief in the pump must close more since more volume is then flowing thru the bearings. A simple proportional control like that does not hold a fixed setpoint. The setpoint droops with the load. The same question comes up as to why the coolant thermostat doesn't hold an exact temperature setpoint.