I believe the oil pressure gauge is just like the fuel gauge in that, if you have an open in the wire, it will read max and if it is shorted, it will read minimum.
I believe with the key on, if you disconnect the wire from the sender, the needle should go to max and if you short the sending unit wire to ground, the needle should go to minimum. If the gauge responds correctly, it is probably the sending unit.
You said "Even pegs when I turn the pump slow with a drill." I assume this means that it goes back to zero when you don't turn the pump with a drill? As I said above, if the needle responds, it's prob the sending unit.