So, this engine?
It looks like the sensor is wired to the cluster directly, at least on the early models. I expect that the sensor works by increasing/decreasing resistance across pin 2- GN/BL wire "oil level" and pin 4- BR/OR wire "analog ground" (Pin 1 and 3 are for oil temperature).
I'd test that theory by measuring resistance between those pins while the sensor is immersed in varying levels of oil. If that's the case, I'd put a dummy resistor in place of the sensor to simulate a 'good' level.