I mean a normal healthy engine might have lower oil pressure, but not where it is tripping the "Check gauges" light at any point during normal operation. Something is definitely wrong. You mentioned that your "chore truck" had a big block.
I'm not sure how similar your engine is to mine, but what RPM does it idle at when hot in Park/Neutral? What about idling in drive?
The factory tach is showing a steady 700 rpm at hot idle. (Which was close enough to the 650 RPM in the manual that I went with it.)
Understand that this is 700 'indicated', not necessarily all that accurate.
Do you have a scan tool to check exactly what rpms it is doing?
Yes I do. I will commit to getting a hot idle RPM reading asap and reporting back with that here
so that you have something to baseline against. (My engine is SES light free & seems pretty happy go lucky.)
Additionally, do you know of any manual that states the proper idle rpms for my engine in those two states?
Here's the Desired Idle speed for the 7.4 per the '99 FSM:
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Speaking of which, you may not be aware that there's a cache of downloadable Factory Service
Manuals covering the range of GMT400 model years. Go give yourself the gift of all the data
that the people who designed your truck pulled together for the dealer technicians to be able to
fix as fast & accurate as possible: (LINK)
After this initial flurry of discovery, we'll get this figured out sooner rather than later.