A couple things to think about. First, oil flow lubricates, not oil pressure (if anyone feels like flaming me think for a minute first). Pressure is the measure of resistence to flow. If your system is really "plugged" it could result in either no pressure reading or a very high pressure reading depending on where the sensor is in relation to the "plug". If you observe oil flow at the rocker arms then the system, or part of the system, is not plugged (though #8 lifter/push rod may be). I think you said you're going to get a "new", known functional gauge and sensor. Since it sounds like you have oil flowing when it's running that is the logical starting point.