Well, it's time to bring this thread to a close. First off, what did I do. I regasketed the oil cooler/oil filter adapter and that seems to have raised the oil pressure regardless of how you are measuring it. I carefully looked at both gaskets and can see nothing wrong. But it is what it is. As to those who mentioned the inaccuracy of the IP gauge, that is the understatement of the century. I installed a permanent second gauge plumbed into the spare port at the top of the oil filter adapter. These are the results.

At an ambient of 75 degrees just after startup:

@1000 rpm, IP gauge = 60 psig, aftermarket gauge = 75 psig
@2000 rpm, IP gauge = 70 psig, aftermarket gauge = 100 psig (100 psig range so this may be higher)


At operating temperature (have no idea what the engine temperature is, just that it stabilized):

@idle, IP gauge = 20, aftermarket gauge = 37 psig
@1000 rpm, IP gauge = 20 psig, aftermarket gauge = 40 psig
@2000 rpm, IP gauge = 40 psig, aftermarket gauge = 75 psig

One thing that I found interesting was that when I let the clutch out and and the engine speed momentarily drops below idle, the IP pressure gauge goes to 0, the bells start chiming, and the aftermarket gauge still shows slightly less than 30 psig.

So, what have I learned?

1) You can perform a self test on the instrument cluster using the trip odometer reset button.
2) All of you guys are right, the IP oil pressure gauge is barely connected to reality.
3) If you do a comparison between the IP gauge and a test gauge, single point comparisons can be misleading. I wire-tied a mechanical gauge I had to the left windshield wiper arm and drove the truck like that for a couple days. Amazing what you find over a range of operation conditions.
4) There is a TSB covering this very problem, TSB 08-22-99. I don't think mine has been reprogrammed as it doesn't have the update decal on the PCM. And before someone asks, I'm not driving 150 miles to have this done when I have the aftermarket gauge.

So, that's that, I consider this matter closed. Thanks to everyone that commented. up

"We live in a time when intelligent people are being silenced so that stupid people won't be offended".