just remember now (what a nights good sleep can do) that on my 2nd gen. B12 engined bike with Slingy loom, I connected the blue wire from the switch to the blue wire from the loom and it worked. Nothing else needs connecting or modifying.
the switch in the tank is very easy to wire up, it only needs one wire to a light/led somewhere near the clocks, and a wire to earth. Another wire from ignition to the light/led and you're done.
the oil in the sump is cooler. The head warms up pretty fast after starting the engine, the crankcases and everything below take longer to warm up. The readings you get could be accurate.