Well, here is the update. I replaced the o-ring, started it up, and my heart sank as I watched the oil pressure again slowly drop to about 10 psi. The difference this time is that it is not dropping to zero and when I increase the RPM to about 2,000 the pressure goes to 30 psi. Before I was getting pressure dropping to below 10 psi, the low oil warning light was coming on, and revving would maybe bring it up a few psi. This time the warning did not come on even after letting it get up to 210 degrees on the temp gauge.
So, maybe my pump is bad or maybe the dealer I got it from put magic mystery goo in it to cover worn bearings and my oil changes flushed it out. I doubt with 170,000 miles the bearings are bad. I just talked to a guy that had a 2005 Yukon Denali that he got rid of because the transmission went out at 300,000 miles.
I have cluster out of a 2005 I may put in for test purposes just to make sure. I think it has bad stepper motors and I was going to fix it anyway. Probably not the gauge since I would assume the low pressure warning would have been triggered by the pressure sensor not the gauge. I am not looking forward to tackling pulling the bottom pulley on that crank.