You know how in some italian dressings they use something that keeps the oil and water mixed evenly so there's always consistent mixture? What's the stuff they use to do that?

You need an emulsifier. Lecithin is commonly used for this in foods.

Mustard and egg yolks* also work as emulsifiers, and are commonly used in dressings.

* contains lecithin

or if you're not planning on eating it: soap.

Emulsion... hmmm... lecithin is the only one I've heard of.

Now, if you wanted a DEmulsifier, we get drums of that stuff every month. Refineries don't like watery crude oil.

Gums, like xantham gum, can be used to obtain the property you are describing. I believe they are used in salad dressing more than things like lecithin. Xantham will increase the viscosity of the product at a very low concentration, along the lines of 0.5 - 1% of XG to product; so it takes a very small amount.