For at least 20 years Ford
oil pressure gauges have been essentially idiot lights. They have a switch at the engine and when above about 5 psi the gauge reads mid scale. Below that they read 0.
This was Ford's solution in the mid 90s when everyone went to lower
oil pressure for better mileage.
GM tweaked their sending units to read a bit higher than actual at that time.
This was all driven by customer complaints that the
oil pressure was "low" on most cars.
I was a GMC Truck product engineer in the early 90s.
John