The high pressure oil pump should run around 65psi running speed and still be around 15-20psi at idle. 20w50 is an appropriate grade of oil. Check for any restricitions in the oil line between the block and the one-shot and the one-shot and the gauge. Air in the line will present as the gauge being slow to react when the car is first turned on (it'll look like no oil pressure for a second until the air is compressed or pushed out). If everything looks good, I'd probably change the oil again and forget about it. If you're really worried about it, put on a new oil pump. Their not too expensive and are easy to install.
