Preservative for Herbal Hair Care Products

I am preparing a formula for hair care and preservation. The products are all natural oil, leaves, petroleum jelly, fragrance. I need to know what if any preservative is necessary, and what quantities should I use.

Herbal hair care products have usually short shelf lives. A typical shampoo, for instance, should be used within one week. This, of course, makes it unsuitable to prepare in larger quantities. For 350 ml shampoo 1 tsp of vodka will extend the storage time to 3 weeks, if kept in the fridge.
Vitamin E is another good choice, in which case 1 tsp is recommended for 350 ml. I am presuming you are using essential oils for fragrance, and the use of have a antibacterial effect. Antioxidants that prevent the spoilage of organic products can also be used in natural cosmetics. To mention a few, vitamins A, C, and E. Horsetail has been used in the past as well as citric acid. We sell a very useful book called, Naturally Healthy Hair, and you might be interested in purchasing it.