Back when I had my suspected lifter problem, I run Marvel Mystery into it, then changed the oil, and then SeaFoamed it. Changed the oil again.

I realized I had my strobe with me, so I just checked the crankspeed - hot engine, no load (in park, no lights, no AC, no fan speed, control set to off, and strobed 657 - put in drive, turn the AC on MAX & Fan Speed #4 (where the problem occurs) and strobed 606.

50 RPM drop when AC & Fan speed Load, combined with being in Drive.

Jiggled the sender wire, no change. This issue is very repeatable. I could swear that just now I could hear some rapid clicking (not engine clicking, too high-pitched and rapid) whenever the gauge needle would drop. Like a solenoid clicking on a low battery, that real rapid clickity click. However, I've got a loose heat shield somewhere along the exhaust pipe that likes to rattle at idle.

I have a 250 mile drive home tomorrow. Is it safe to assume that if it's simply a bad oil pressure switch, it won't affect driveability, but I'll be running without a warning system until I can change the sender switch out ?

Thanks for the reply !