[84LX89GT]

Oil is thicker when it's cold and when it warms up and gets thinner it doesn't fill the clearances of the engine as well (if it's excessive). Chances are your main and rod bearings are worn and need replacement. Changing just the oil pump won't fix your problem unless you get a high volume, which it MAY fill up the gaps better - but if you're in there check and replace the bearings. A temporary solution is running thicker oil like 15w40, 20W50 or even a straight weight like 30, or 40 (wouldn't recommend straight 50 if you plan on actually driving it) and your oil pressure will go up. I had the same problem in my '87 Jeep Cherokee and i went up in oil viscosity (from 5W30 to 15W40) and the oil pressure came up to where it should have been.