In my opinion, any reading below the green should be investigated.  Have your mechanic hook up a direct reading gage to the oil pressure line coming off the engine and see what the actual oil pressure is.  If the direct reading gage reflects what your in dash gage reads, then you have a problem.  Check the oil pressure relief valve, make sure the ball is s eating properly, there are no worn spots on the seat, the spring is not weak.  IF all those are good, try adjusting the oil pressure relief valve to increase oil pressure, usually this is by turning the but clockwise to put more spring pressure on the ball bearing.  If you have an older engine, the old style relief valve is adjusted by adding washers in the valve housing.