The first thing I would do is connect a mechanical gauge to the engine and verify the gauge in the car is reading correctly. If they are within 5 psi of each other at idle and revved up, note if the gauge in the car is a little high or a little low.
Even assuming your gauge is correct, 20 psi at idle wouldn't bother me, as long as the pressure goes up to 50 or more at high rpm. 15 psi at idle might not actually be a problem but that is getting low. I have had old worn out engines idle clear down to 5 psi and still run another 50K miles, with no problems. If your pump is the standard volume pump, 20ish is normal. If it is a high volume pump, I would expect more, but not be overly concerned.
Do not go to 10-30 if the pressure is already low with 10-40.
If you have low
oil pressure odds are that it is something other than the
oil pump.
Oil pumps rarely go bad unless they are starved for oil. I would be making sure any external oil filter plumbing or oil cooler was done correctly.
Usually low oil pressure is caused by loose bearing clearances. When I was a puppy, the racing crowd purposely set bearing clearances loose an ran heavy oil. Now the trend is to set bearings tight and run thin oil. I think the latter is better, but the fact is both strategies work. If my oil pressure dropped below 15 at an idle, I would put in heavier oil and run it, as long as there were no other signs of problems.