The sending unit is a strain gauge. A rubber diaphragm with a resistive element glued to it. As the pressure from the oil stretches the rubber, the resistance decreases. If you pull the wire off the sensor and short it to the block, the gauge should go to full range (ignition on or gauges powered) If not, it is the gauge. If so, it is the sending unit. They are calibrated differently for different engines, and versions of the same engine. They are a high failure item due to heat, pressure and vibration. Gauges rarely fail, sending units fail frequently. In a pinch, anything that fits will do, all you really care about is notification of a total loss of oil pressure, not a constant, accurate or relative measurement. My Toyota requires over 100 psi of oil pressure to actuate the advance mechanism on all 4 camshafts based on engine load. The sending unit screws up regularly, I am on my 4th. No biggie, it has nothing to do with engine condition, it's just an indicator.
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