Lube light was discontinued in 1968 with the "new models" - image above is from the repair manual. And yes, the wiring diagram does show the light in a separate box on the same page. If I owned an SOS, I would install one and make sure it works!
Testing of the circuit is backwards from what you describe. The light is designed to turn on with low pressure, therefore resistance should be zero with the engine off, and vice versa. That said, the last SOS I worked on had a faulty switch, which I had to source aftermarket.