Question 1 from 4th Jan; Use only one grit on the zinc or zinc+ disk. You can change it, but only from finer to courser grind. For instance, you can go from 8k to 3k on it, but you shouldn’t go from 3k to 8 k. I mark the obverse side of my disks with the grit and store them separately to avoid cross contamination.
Question 2. Oils are a preference. I have used coconut cooking oil as an extender with good results, then washed the disk and added pandamonium. If you change oils, just wash so they don’t react to each other and form blobs.