I got a chance to work on this some more today. I put a mechanical gauge (the one that replaces the oil filter) and the cold oil pressure went up to 60. I am not sure if that is due to no restriction of the filter or what, but the mechanical gauge and the dash gauge matched. As the truck started getting warmer the pressure dropped on both gauges, but now the mechanical gauge was bouncing very rapidly between 15 & 30 PSI while the dash gauge was steady. Once it reached operating temp the dash gauge was at zero again and the check gauges light was on however the mech. gauge stayed bouncing between 15 & 30. While under there I noticed the truck had the typical leak where the oil pan and timing cover meet so I dropped the oil pan. There was some large chunks of sludge in the bottom of the pan so I am going to degrease everything and go ahead and replace the oil pump while I am there. I am also going to remove the valve covers to make sure the oil return passages are clear. If this doesn't fix it then I guess it is just a tired motor - I can't tell by the way it drives though it has plenty of power and sounds like a new one running.