Old school makeup wisdom would say that if you have oily skin, you should ONLY use matte foundations. Matte foundations can be great because you obviously don't want to add shine to your skin, but they can also go TOO matte and start to crack and break apart as the day goes on. As a pro makeup artist, I actually use non-matte foundations on oily skin often. The key is to use a mattifying primer first (I live for this one by Hourglass Cosmetics), and then a great setting powder that you touchup with throughout the day. This doesn't mean all foundations will suddenly work great on oily skin — you definitely want to avoid anything labelled "luminous", "hydrating", or "dewy", or that you find makes your skin feel greasier. The best way to know if you'll like a foundation is to go to a big beauty store like Ulta or Sephora and get samples! Ask the employees there for foundations that have a more "natural finish" rather than matte or luminous, and then take them for a test drive for a full day. Foundation always looks great when you first try it on, but you want to give it a full day to see how it really pans out with your skin.