5 psi at idle is enough,but as soon as the engine speed is increased,the pressure should go up to at least 25 psi when warm.These engines will run for a long time with low oil pressure if ya don't push hard.
A 235/261 with a good oil pump and proper bearing clearances will have when hot,30-35 psi at idle and 40-45 psi going down the road.
If your engine has such low oil pressure,it either isn't rebuilt or was a quicky job or the machinist booted the clearances.Use a heavy enough oil to maintain a reasonable oil psi when hot.
