If a formula is totally free of water but consists only of oils, you don't necessarily have to add a preservative. Microbes including bacteria and mold require water to grow. However, in order to prevent the oils from becoming rancid (= oxidation) you'd have to add a good antioxidant such as, for example, Vitamin E Tocopherol. If you still prefer to add an antimicrobial agent I'd recommend using an oil-soluble ingredient, like tea tree oil or Orange-Lemongrass Blend (http://www.makingcosmetics.com/). Both are used typically at a concentration of about 2% (= 2ml per 100ml of oils).