Low oil pressure at warm idle

Get a inexpensive mechanical oil pressure gauge (equus/autogauge) from Oreillys or some place and temporarily install it in place of the stock sender.
This should verify the pressures that your digital sender is seeing.

I use a 1/8" BSP nipple and T and use two senders on my SS build, one is for the stock meter, one is a autometer unit in the gauge pod.

Add half of a quart of atf to your oil and take the car for about a hour drive then change your oil and see if it still happens. The atf is a engine safe cleaner that will clean oil sludge and residue that can cause oiling issues.

Or Seafoam if you have some of that laying around. 1.5oz per quart of oil, change oil/filter 50-100 miles after adding to oil.

I wouldn't be trying treatments on a 138k motor. That's a recipe for losing your ring seal. Definitely put in a real oil pressure gauge like Z_karma suggests but change to a 15w50 or 20w50 that's meant for gas cars, not big diesels or bikes. If that doesn't work you're going to be ripping into it anyway.

edit for clarity: at that many years and miles, carbon and sludge are part of your motor and are helping with compression and all of your oil seals. This is totally normal and doesn't mean your motor's on its way out, you can cruise along for another 100K like that as long as nothing happens to change it. ATF or Seafoam will break that stuff loose, which at this point is likely to cause more problems than it solves (bad ring seal, oil seal failure, maybe even blocked oil passages.)

If the heavier oil gets your pressure up, which i bet it will, the best solution for slowing further carbon and sludge buildup is 3500-mile oil changes with frequent Italian tuneups.

People do throw ATF and seafoam in their crankcase but they usually do it more regularly and started much earlier on in the motor's life. I just don't think it'll help the OP. He's probably just got an old motor and Texas summer heat thinning his oil out.

Not to start an oil thread / go too far off topic.. Sludge and carbon build up are bad for your low mileage 138k engine. I use seafoam in my mid mileage 200k engine to help "thin" the oil a little before I do an oil & filter change to try and get as much of the old oil out as possible. Why? So the new oil / filter & engine will stay cleaner longer.

On topic, I agree with Z_Karma, just get an oil pressure tester, they are available all over for $25, Amazon, Harbor Freight, Walmart, etc, etc. then you'll know for sure.

I appreciate all the responses. I already changed to 2w50 oil and this didn't fix the issue. I haven't run seafoam through the car since this issue started. Ran some through it when I first got it in 2015. I do have a mechanical gauge that I need to put in, but am waiting for the weather to cool down a little to install it. On top of this issue, it looks like the power supply for the dash has gone out in the last day or so. Guess I will be looking for one of those now, or maybe baking the existing one to melt the solder. All part of owning a 33 year old car. Lol

Edit: I took the power supply out and reflowed the solder on most of the connections. Dash is back to working the way it was previously. Still no display on the fuel gauge though.