So, when you connect wire to the ground (simulating what the oil pressure swtich would do on low oil pressure) and the lights do not light, what you have done is proved that the oil swtich is not the problem. Beacuse you did what the swtich should do and the rest of the system didnt work, so the problem is somewwhere in the rest of the system.
Suppose to be power (from instrument fuse) to one side of the the oil light all the time, from the other side of the light a wire goes to the swtich, and the switch then either connects the wire to ground or does not connect the wire to ground to light the light or not.
THe sidestand light is the same and has its own switch to ground, and the netural light is the same and has its own switch to ground.
You said the problem is that the instrument lights and the oil opressure and the sidestand light and the netiural light and other things are not working all at once, then they do all work all at once. THe only thing common to all those are not any one ground, but the power to them all. You should be checking the power wire from the fuse to the instrument panel.
I am repeating myself. I will quit trying to explain this.
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