From what I can tell from reading a number of sources on the web, Soybean oil, Canola oil, etc that are used in "organic" based pesticides are food grade forms of those oils. If that is correct, then those types of oil sold for cooking should be ok! for use in "homemade" pesticides.
Water seems to be the main ingredient used in "organic" oil based pesticides eg
Oil Pharm - Download RTU Label
Active Ingredients: Certified Organic Soybean Oil 1.5%
Pure Rosemary Oil 0.1%
Inert Ingredients: Food Grade Oleic Acid, Carbonic Acid Monopotassium Salt, Non GMO Canola Oil, Broccoli Juice: 10%, Water 88.4%
Using Oils As Pesticides (download/read pdf_1192.pdf on that page) from Texas A&M University
Oil formulations are generally designed to be mixed with water at concentrations of 0.5-2.0 percent (volume/volume). When applying oils, it is best to agitate hand pump sprayers frequently and keep tank spray agitators running. This reduces the risk of oil separation that could result in sprayer clogging, uneven plant coverage and possible plant injury.
A little bit of Soybean information to be aware of, if in fact it's true:
In the Aug 2010 issue of ABJ, Zachary Huang discusses bee nutrition. He points out that about 40% of the sugars in soybeans are toxic to bees. He goes on to say that Brood Builder® and Bee Pro® are soy based. He doesn’t specifically state that they aren’t OK but doesn’t include them in any of his analysis. Take it for what it’s worth.
Then this is worth reading and watching, "Are you eating pesticides? Canola oil, soybean oil used as key ingredients in pesticide products", but only because of the amount of "basic" information it leaves out, thus supporting "their argument". Personally I might go as far as saying, "their argument" borders on scaremonger...