I would first get an alternate gauge borrow one etc and see if there is a difference, if not I would worry there is a clearance issue somewhere crank rod cam etc possible oil gallery is cracked I have seen several things that cause this. Oil pumps are cheap probly 30 bucks can score you a hp hv melling unit a new screen and a pinned drive rod, note a lot of people would say to stick with a stocker because it take less hp to drive it essentially more horse power, I have had good luck with the stock replacement it should hold a good 40psi unless one of the aboves is a problem. just my 2 cents I have built a few chevy motors before and have owned several with teribble oil pressure and most of the time its crank and cam bearings that cause it and one that had a bunch of orange formagasket stuck in the pick up too, but mostly clearance once hot lets the oil just fly past it starving the rest of the motor.