If the relief valve is stuck, or severely worn, it should be apparent upon dis-assembly. Take it out and press on the plunger gently. The plunger should travel back into the relief housing body, so that it clears the relief holes on the sides. If it sticks or binds up, then its likely sticking during operation, and causing low oil pressure. If it operates smoothly, then I’m sorry to say that its unlikely that its the main cause of the low oil pressure condition.
A simple modification can be done to increase oil pressure, but this will only be successful if there’s a healthy pump and healthy engine clearances.
You’ll need to ask yourself what oil you have been running, and exactly what was done 28 thousand miles ago. If the cam bearings were not done at that time, its a likely culprit. The Alpine engine is designed to all wear out evenly through its life. Pump, Crank Shells, and Cam bearings. (Refreshing bearing shells on the crank does not an overhaul make.)