I can tell you that the original oil pressure sender and the replacement (a Sierra Marine
part) both measure 260 ohms when not installed on the engine
I purchased a Home Depot 0-100psi mechanical gauge (from the well pump department) and an 1/8NPT nipple and coupler, and temporarily installed that to make absolutely certain that the problem was not with the engine itself. When I started the engine the gauge gyrated wildly around 50psi as a result of it not being fluid damped. But that was okay. It told me what I needed to know.
So I reinstalled my new sender and fired the engine up again. The gauge reads 75psi (remember the gauge reads 25psi when the engine is off and the ignition is on). So it has to be something with the gauge itself.
Valiant 40 #163