That is small but very crucial detail that you should have mentioned in your first post. Where you measure pressure in a hydraulic system matters a lot and it did not occur to me that your pressure gauge was not connected to the normal pressure tap between the pump and the relief valve. It's as if you're concerned about the water pressure in your house but you've put your pressure gauge at the end of a garden hose.
If you're still using your oil pressure switch, add a tee to pressure port and move your oil pressure gauge to it. With a high volume oil pump, my guess is that your oil pressure will be constant at all times because it will most likely be always at the relief valve pressure setting and oil will be constantly relieving back to the sump. I would try a lower viscosity oil like a 10W-30 API CK-4 HDEO (with a Ford WSS-M2C171-F1 approval), which will get more oil to the bearings and less to the sump. I like PetroCanada Duron SHP 10W-30.