[Decolliber]

Most likely the sender needs to be replaced. It looks easy but is not. Mine could not be unscrewed. It screws into a block and I had to remove the block and put it into a vice. To remove the block you need to remove the oil lines that also connect to it. (Good time to replace those lines if they are leaking). The sender is behind the fan belt.
You can check whether the oil pressure guage is faulty by running a current through it with a variable resistor ($2 at Radio Shack) in the line. I was able to make my guage go through the full range of movement by varying the resistance, so I then knew the problem was either the sender or a faulty wire between the guage and the sender.