Going off my memory here...all the sender (switch) does is supplly a ground at about 7 psi oil pressure. That's not very much! When the ground is made, a full 12 volts would be applied to the gauge and it would read full scale, except along the way there's a 20 ohm resistor that provides a voltage drop. This brings the needle on the gauge down to some mid-range value that would look something like a normal pressure reading. I've had my instrument cluster off (for other problems) and I've seen that resistor soldered on the back of the cluster.
Try this...just unplug the wire going to the sender and jumper it to a good ground, anywhere on the engine. If you have a normal indication, then you have a faulty sender, even though it's new. If not, you have a resistance fault in the circuit somewhere. If you had an actual pressure problem, then the pressure would have to be hovering right at 7psi, which I would think would be VERY unlikely. To make absolutely sure, you'd have to purchase and install a direct reading pressure gauge.
All this, of course, assumes that there's been no modifications to the circuit!!!