Not sure if you have tested your current sender yet, but if you look in the old Van's instructions for that oil pressure sender, there is a test procedure. Search the inter webs for VOP-100, you will find this document
I just recently had a problem with oil pressure readings, measured with a mechanical gauge that showed it was either the sender or the dash gauge. I did the test procedure and determined my sender was good, but my gauge had a "sticky" spot or something. I connected the wire that is normally on the sender to ground, and then powered up my gauge with the master switch. It came up to about 60psi, but with the wire grounded, should read max pressure.
I continually power/unpowered the gauge using the circuit breaker for that gauge. Each time I did so, it would go up to a higher reading, until it suddenly went to full max reading (which on the old van's gauge is above the red line). That is what it should show according to the test procedure, when grounded.
After that, it went full travel easily, and I hooked everything back up, and started the engine, oil pressure read normal, have not had trouble since.