A pump is just a device that moves fluid. If you cause a restriction of that fluid or if the pump moves more volume that is being bleed off thru clearances it creates pressure. To keep this pressure in check you need a relief valve. If your clearances (because of wear) start to match the pump output the pressure will start to dropping. Putting a higher pressure spring with the same volume pump will do nothing to add more pressure because the pressure is already below the current pressure relief setting. The only way to raise the pressure with the just a pump is add more volume. More fluid in with the same fluid out will net more pressure up until it hits the pressure relief setting.
As far as high volume pumps running the pan dry is just an urban legend. If you can only move so much fluid thru the motor and you add a high volume pump the oil ends up back into the pan because that is were the pressure relief valve dumps. You are just waisting motion by loading the pump and cam gear moving fluid thru the pressure relief.
If you are loosing oil pressure steadily you have something wearing, could be the engine or could be the pump. On a LA you have to pull the pan to change the pump so I would pull the main and rod bearings to take a look.
I will tell you that 20psi at idle I wouldn't sweat. Remember if it is a positive pressure you are moving more fluid that the clearances are consuming. I would be worried more if it doesn't stabilize because it would mean somethings going away.
Chuck