probably because is a universal nismo cooler. and it wont show a pressure difference because its not under pressure, its an open return into the pan. the restrictor helps hold oil in the cooler so it doesnt flow through to fast and has time to cool.
id modify the system with a sandwich to return to the sandwich and not the pan for this has been discussed to help cut aeration and offer full time oil cooling not rpm/pressure based operation. as the factory tree opens the valve to the cooler system and bleeds off oil that could be pushed into the primary system if a sandwich style system is used in place.
