Rally dash oil pressure gauge issue.
I think you may have two problems here:
1. The gauge reads half scale with 10 ohm connected at the tank. This indicates that there is a fault in the wiring or gauge probably. Could be extra resistance somewhere, or a bad gauge.
If you temp gauge is working, your IVR is probably ok.
2. The fuel tank sender, which reads 10 ohms with a meter, gives a 1/4 scale reading. It should read the same as the 10 ohm resistor. This likely means you have a bad ground on the tank, or a bad connection to the tank.
The above assumes I have stated your test results accurately.