I stumbled across these forums the other night, and have noticed quite a few misconceptions, especially with regards to the "oil and Fleshlights" topic. I'm a biochemist, so I'll try to clear up some of the issues surrounding oil-based lubes and Fleshlight inserts.
Why not oil?
The reason not to use oil-based lubes is that they have the capability of dissolving molecules out of the Fleshlight insert. The polymer materials used to make the insert are oil-based by nature, so it's best if you use a lube that is repelled, on a molecular level, from the insert, rather than being able to mix with, and hence dissolve it.
What is an oil-based lube?
An oil-based lube is defined as having free oil capable of dissolving other oil-based substances. The two basic types of oil-based lube are oil solutions and oil emulsions. Oil solutions have a large amount of oil dissolving other oily ingredients. This would include petrolatum (petroleum jelly, Vaseline) products. (Please note that Vaseline is merely a brand name, and several water-based products, such as certain lotions and cremes, are available under that brand name.) Oil emulsions have a mixture of oil and water in which either the oil or the water is broken up into tiny droplets and mixed into the other. Both of these provide oils capable of dissolving more substances, such as molecules from your Fleshlight.
What oil is okay? Why?
Certain ingredients of water-based lubes are okay, even though they are labeled as oils. Particularly, almost anything from the final twenty or thirty percent of the ingredients list isn't present in enough quantity to do anything to the FL (ingredient lists are organized from most plentiful to least plentiful). As well, essential oils, used in many water-based lubes and alternative lubes (such as conditioners) are harmless. These essential oils include tea-tree (Melaleuca alternifolia) oil (a chapped-skin treatment, so is great for regular FLyers), clove oil (a desensitizer, so it's mostly used in partner intercourse to aid in fending off premature ejaculation), and pretty much any other plant-named oil (eucalyptus oil, menthol, kangaroo's paw oil, et cetera).
The reason these ingredients do not harm polymer substances such as FL inserts is that they do not provide any free, available-as-a-solvent oil. While pure essential oils are excellent solvents, used in water-based products they must be coupled with other substances which will both dissolve the oil and itself dissolve in water. Since each oil molecule can only associate with other oil molecules (or oily parts of other large molecules), the mere presence of an oil in a water-based product (i.e. one that does not form a film of free oil when dissolved 50% in water) means that the oil has been "used up," and is no longer free to dissolve other substances -- if it were, it wouldn't stay dissolved in the water-based medium.
I've also noticed a few misconceptions about glycerine, from thoughts that it's some kind of oil, to that it's made from "hydrolized [sic] mineral oil". Glycerine is derived from fat,meaning vegetable oils and animal fats, not mineral oils. Glycerine is, in fact, the non-oily backbone fats are built around. A glycerine molecule will have three fatty acid (natural oil) molecules attached to it naturally. In the process of making soaps from such fats, the glycerine, not being oily, separates from the oils.
Far from being oil, glycerine is even less oily than other alcohols (a large, and mostly non-stinging family of chemicals: just because something says "alcohol," don't assume it'll sting or dry your skin out). It's actually more like a sugar than what you would normally think of as an alcohol. (Though sugars can be simply thought of as alcohols with formaldehyde or acetone tacked onto the end . . . ) Glycerine is actually one of the most water-soluble substances there is, and will actually pull water out of the air if it's not sealed tightly. Needless to say, it's marvelous in FL lubes, and won't bring the slightest bit of harm to the insert.
Shampoos and Soaps
Some people have wondered why shampoos and soaps (including the so-called "shower gels," which are actually just shampoo repackaged as soap) damage polymers. Even though they don't seem to have oil in them, shampoos and soaps (generically referred to as detergents) are half oil and half water in their personality. Their utility actually stems from the fact that they're able to mix with both oil and water at different ends of their molecules. This allows them to lift oils off of dirty hands/clothes/vaseline-soaked cocks, et cetera, and into water to be rinsed away. This also allows them to pull molecules out of polymer materials, breaking them down even more quickly than most oils.
As far as the vegetable-oil controversy goes (since the company does mention it, I thought I'd address it): vegetable oils are very poor solvents. Since their molecules spend so much time just interacting with each other, they have little time to "bring in" other molecules. You might call them cliquish. In fact, as long as you don't use an oil capable of going rancid or drying out (such as corn oil or safflower oil) and stick to the "sweet" oils such as olive oil, canola oil, and grapeseed oil, you and your FL should be just fine. That's great news for those who like the unparalleled slickness of an oil but don't have the money to replace their FL insert every couple of months.
Oh, and a word about WD-40: it'll dissolve your FL insert faster than most anything else you might think of to use as a lube. I was rather surprised to see a couple of threads in the archives on this subject, and thought I'd add my own word of warning. WD-40 IS AN OIL-BASED SOLVENT, MUCH WORSE THAN A SIMPLE OIL-BASED LUBE!
So, I hope you know a bit more about the chemistry of slippery wetness than you did before . . . and go ahead, experiment with the latest "Made with 17 kinds of aromatherapy essential oils" conditioners if you want to . . . just mix a bit half-and-half with water first to see if there's any free oil in them. (Mixing more dilute than that, unless you actually use it more dilute, will get water between oils and their associated molecules, freeing them.)