To answer the 2nd part of your question, usually the sending unit works on resistance and you can check to see if it is operating with a common handheld meter (using the ohm setting). More or less oil pressure will cause the gauge to receive different resistance values. But there are a couple of different kinds. Some also output different voltage amounts. More or less oil pressure will cause higher or lower voltage values. As mentioned above, we really need to know which gauge you have. Note: some of the AutoMeter sending units can be pretty costly.
