We naturally have oil in our hair, but we wash it away with shampoo. Hence the need for conditioner to re-coat hair with something smooth and slippery.
You could just use oil as a conditioner. You can buy oil sold for that, and you can find recommendations to use olive or other food oil as a conditioner. But it's hard to avoid ending up with very heavily greased hair, unless you oil it first and then wash the oil out.
With a small amount of oil diluted in a larger amount of water, it becomes easier to distribute a small amount of oil through your hair. But you need a way to get the oil and water to mix. The xanthan gum and guar gum make that possible, but they have other functions as well:
The xanthan gum and guar gum:
1) Make it possible to mix the oil and water. I don't know how much of that is just a result of their thickening effect slowing the rise of oil to the top, and how much is a true emulsification effect, but in any case it seems to work.
2) Their thickening effect is also useful in creating a thick gel that can help get your hair to go where you want it to.
3) A thicker liquid is easier to apply--it will stick to a comb, and less of it runs between your fingers and down the drain when you try to use it in the shower.
4) In addition to making liquids thick, they also make them super slippery, to make combing easier.
You can read more about xanthan gum
and about guar gum in Wikipedia
I tried both olive oil and canola oil. The canola oil seemed to give a lighter feel, which I preferred, but the olive oil smells good and might tame your hair a little more if that's what you want. I think canola oil will last longer before it goes rancid. Jojoba oil is a vegetable oil that is mainly used in cosmetics. It might be a better choice, both because it works well and because it doesn't go bad. But I thought it was more convenient to use something I have on hand anyway and can get in the grocery store.