I'm with Jerry, Excessive bearing clearance.
Oil pumps never seem to fix low
oil pressure problems. I used to have a 71 F100 with a 390. It had 3.73 gears
and a hot cam. I loved revving it out but it had almost no
oil pressure (2 PSI maybe) when hot at idle. The factory oil light would flicker at stop lights. I drove the heck out of it for two years with no problem before I replaced the crank and rod bearings.
I would just keep driving your car and not worry about it. If this makes you feel any better check out Chevrolet's minimum oil pressure specs. for new truck.
1000 RPM-6PSI min.
2000 RPM-18PSI min.
4000 RPM-24PSI min.