Oil Pressure

["Dave Martin" <dwmartin@domain.elided>]

Dear Listers,
I need some help with understanding what's happening with the oil pressure
in my 87 Spider Quadrifoglio.
I've had it 11 years / 77000 miles (it just turned 100k). I've always used
Castrol 20w50, and normal pressure is about the 86 psi mark.  Friday I
changed the oil and filter - couldn't find Castrol at the store and so used
Quaker State 20w50 instead (I would hope that the brand is not a factor but
I'm going for full disclosure...).  Saturday I left on a 500+ mile trip.
About 200 miles into the trip, suddenly noticed the oil pressure was ...
HIGH.  It was almost at the top-of-scale 114 psi mark - call it 107.  No
other indications of trouble - no noises or smells, temp was fine.  Stop and
check - nothing unusual under the hood, and oil level is where it was after
the oil change the day before.  At idle the pressure drops to mid-scale,
about 57 psi (which is about double the normal idle pressure).  Later in the
trip, the pressure sometimes drops to 86 (the previous normal), then drops
further to about 57, then climbs ... By end of trip it was again holding
steady at about 107.
I'm thinking/hoping it is not a problem with the actual pressure, but just a
bad gauge or sensor.  Anybody have another explanation? Anybody know how to
troubleshoot and tell whether it is gauge or sensor?  And, oh by the way,
where is the sensor?
Thanxinadvance for your help,
Dave Martin
Endwell, NY
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