I've done some research for natural preservatives to add and found the following seem to be the best:
1- Rosemary Leaf Extract: Seems to be the best totally natural one. Some others should work way less or don't be natural. (For example the famous grapefruit seed extract it seems it acts as a preservative cause of the actual artificial preservatives used in the making)
2- Natural Vitamin E: Good preservative for some bacteria (not general) and good for the skin. (Not all "natural" product brands sell the natural vitamin, make sure it's natural, only natural works as a preservative.)
3- Potassium Sorbate: Not as natural as the above but lot of comercial "natural lubes" use it. Good to prevent mold and bacteria. In theory, way less side effects than most other preservatives.
I bought some rosemary and will try it with xanthan and water alone in a month or so. Let's see...
I don't have or plan to a fleshlight. I haven't done xlube yet (will soon). But i'm interested in xanthan lube and this forum seems the more active about it. If anybody had experience with the components or whatever you'll tell. I wrote it to share and for anybody interested to know and test.