Most of the scores we see for comedogenic ingredients come out of the rabbit ear assay or rabbit ear model, which is a test in which "the test material or an extract is applied directly to intact and abraded sites on the skin of a rabbit. After a 24-hour exposure, the material is removed and the sites are scored for erythema and edema." (redness and swelling). This link. As in the name, the tests tend to be done on the rabbits' ears.
There is a lot of controversy about the rabbit ear model and its application for human skin. As this article notes, this debate is not new as different models show very different comedogenicity levels for the same ingredient. This textbook notes that "Lists of comedogenic ingredients are not necessarily meaningful" because they cannot predict the comedogenicity of the final product as the concentrations used on in tests aren't the same as those used in a product, like a lotion. We've seen how some ingredients can lower the comedogenicity level of other ingredients - for instance, using 1% to 10% mineral oil with IPM can reduce its level from 3.6 (out of 4) to 1.3 and 25% can reduce it to 1 - and we've seen that some ingredients are considered really comedogenic on one scale and not at all on another, like shea butter.
Having said this, rabbit ear testing is being phased out or has been forbidden (EU) and we're seeing more testing by biopsying the back skin of human volunteers who have been shown to form comedodones easily. (Reference). It's hoped this will produce more accurate scales of comedogenicity.
Is mineral oil comedogenic? Not according to this study. "Greasiness cannot be equated with comedgenicity. The admonition forces acne patients to seek "oil-free" skin care products has no scientific merit with regards to comedogenicity. Cosmetics are an unlikely cause of the relatively high prevalence of post-adolescent female acne." Wow! That's pretty amazing, eh? Now, this was published in 1996 and there may have been some studies done since then, but I think it's a pretty amazing study in itself! Check out also this study Is Mineral Oil Comedogenic? which came to the same conclusions. Apparently the perception that petroleum products were comedogenic came from a time when the ingredient was poorly purified and traces of tar - which is comedogenic - would be found. This doesn't happen any more.
What does all of this mean? Keep an eye out for those ingredients that seem to make your skin worse! If shea butter is listed as a 0, it doesn't mean that it won't cause you problems, and wheat germ oil (a 5) won't necessarily bother your skin. Keep a record of these ingredients. This is one of the reasons I suggest keeping your initial formulations simple - using five different oils might seem awesome, but if you break out or hate it, it's hard to figure out the culprit.
Also keep in mind that comedogenicity isn't just about oils. Ingredients like surfactants or humectants can cause issues as well. (See this study on the safety of sodium PCA - it is not considered comedogenic.) Some studies I've seen suggest that harsh detergent based cleansers can be worse for acne prone skin than applying oils!