I "misremembered" about the ground wire. Lesson: don't trust the memory so much when you're over 40. It's installed, and I double-checked continuity. My panel originally came from a panel shop without that ground wire, but I noticed the omission and installed it long ago. Note that I have never had any fluctuating reading issues with fuel, pressure, etc. Only in the past 40-50 hours have I had periodic oil temp issues. I've replaced the probe and more recently the vernatherm. So it was with a new probe and new vernatherm that I noticed that pulling the breaker on the AF3500 causes a magically instant 20 degree immediate change in oil temp.

Unfortunately, I guess I'm back to the drawing board on trouble-shooting oil temp.
At least I can avoid the funky reboot failure, though you guys should eventually try to fix that.