Here is some more info from the '90 shop manual:

Oil sensor:
Engine stopped - 110 to 130 ohms
Engine running -   13 to 55 ohms

Your measured resistance with the engine off is right at the median and the
running measurement at 3,000 RPM should put it shy of the 30 lb/in2 mark.

Oil Gage (as function of a fixed resistance value at the sensor terminal):
0 lb/in2 (at the mark, not at the zero itself)  52 ohms
30 lb/in2 mark  41 ohms
90 lb/in2 mark  16 ohms

I suggest you check the gage. One way of doing this is putting a 10 ohm
resistor in series with a 75 or 100 ohm potentiometer in place of the
sensor. Adjust the pot while an assistant watches the gage position. When
you adjust to any of the points, remove the pot (carefully) and measure the
resistance. Add 10 to the value (or measure the pot and resistor
together)and see how that correlates to the values in the table. I don't
know what the current is, so I suggest you use a larger pot that won't get
too hot. The purpose of adding the 10 ohm resistor is to limit the current
so you don't toast the pot or the gage.

If you decide to measure the oil pressure with a gage at the location of the
sensor, the manual says:

1,000 RPM - 28 to 43 PSI
3,000 RPM - 43 to 57 PSI

As you can see, there is a lot of inherent variance in the readings. This,
of course, does not explain the step change you have seen.

This is probably not a poor connection. A 25% negative change requires an
increase of resistance of around 9 ohms. This is quite a lot for wiring, it
is not likely that you would see this type of change without the circuit
being very intermittent. There is probably not a lot of damping in a gage
such as this, so fluxuations should be visible with a 9 ohm intermittent.

So it's likely the sensor or something mechanical with the engine. If the
sensor has gone south, my preference would be to replace it and continue to
use the factory gage.

Have at it!

Ken