Should the rack & pinion be lubricated periodically? If so, with what?
Every 6 month i clean down and apply a light smear of a very light machine grease.
Couldn't tell you what it is - we had it for 30 year or more! lol
We do the same for all our machinery.
The z-axis is subject to wear so you need to be sure you don't leave it covered in something sticky that picks up dirt and exacerbates the situation. I occasionally gives ours a clean with a cotton bud and a little light oil to make sure nothing it caked on, especially the slide so it runs freely. While you are doing this you can take a nice look at the condition of the gearing, when it goes, you'll know about it.