OK, oops!! if I got them backwards sorry. Ground the lead first, turn the key on. The gauge should read zero. If not, problem is in wiring or gauge itself. If open circuit produces max pressure reading, then resistance is scaled from high to low, ie open=infinite resistance=max reading, not open=intermediate resistance=some other in between gauge reading, grounded=no resistance=zero gauge reading. If you get this response from the gauge, then it's wiring or the sender.
BTW, engines do not hold oil pressure after they are shut off and yes, it is possible for the sender to be defective that quickly, especially if there is some wiring gremlin at work in there..
2000 Dodge Grand Caravan SE
[This message has been edited by jeffsr (edited 09-13-2000).]