Great question. I read somewhere, and I’ve been using ATF (Automatic Transmission Fluid). It is lightweight, doesn’t seem to collect too much dust/crap. Every once-and-a while, I will put a solvent (denatured alcohol) on a rag and hold it against the screw as I manually drive the axis end-to-end, then re-apply ATF. I have a bottle with a thin needle tip on it. I would imagine any lightweight oil, like 3-in-1, would be good also.
Regarding the X-Axis screw under the table, I have to lift the table under one side to get my hand under there. That is to say, it is accessible, but from underneath.