IO's got it. The valve is a pressure regulator, not a safety valve.
If the valve has been messed about with (typically screwed down to camoflage engine wear!!!) You can usually spot this with a cold engine. The oil pressure will be roughly proportional to the engine speed - until you reach a point at which the valve lifts.
I've been through this process with a friend's aircraft - where after a fair bit of work being carried out the oil pressure was very high. In that case even when hot the oil pressure was proportional to engine speed. A couple fo minutes with the manual showed that the engine had aquired a number of spacers below the cap......