So today I had the opportunity to deal with the oil pressure warning
system on a 2.1. Unsuccessfully I'm afraid.  Here are the details:

Email from a WetWesties member dead at the side of the road in my part
of the world. Blinking oil light and buzzer. '89 Syncro. Initial
indications while climbing a long grade. They noted that the oil level
was a bit high so they had the oil drained and correctly filled here
in La Grande, OR before continuing on their way. 20 miles out of town
at the top of a long grade, the oil pressure light starts blinking and
the buzzer goes off.  They shut it down and call a tow back to La
Grande.  Holler for help on the WetWesties list.  I gotta go, ya know?

Fast forward three hours. Both oil pressure switches replaced. 5W30
oil drained and replaced with 3.5 quarts of 15W40 Delo. No lights, no
buzzer, no leaks.  Off these nice folks go on their trip to
Yellowstone. Thirty minutes later, the cell goes off. Clif & Amy.
Same place, same symptoms. Want advise. I dunno. Chat further and find
out that there is no lifter clatter at any engine speed. Above 3k rpm
the light and buzzer go off.  300 miles home, 100 miles to Boise and
Wagonworks. Tough call. So....  Since I really don't understand how
the later model oil pressure warning system works, I'm not sure
whether they have a real engine problem or an indicator problem.

Can the wisdom of the lists bring me up to speed on how the 2.1 OP
warning system works.  Oil pressure problem or sensor problem?

Thanks in advance.

Jim