Carrier oils are beneficial for the hair and skin due to the nourishment and moisture that is provided in abundance. There are numerous carrier oils that are extremely heavy, in terms of, thickness and heaviness which is great for dry hair, yet, there are carrier oils that are considered to be "dry" or said to have "astringent" qualities. These types of carrier oils have a light or dry feel on the skin and absorbs very quickly due to the many tannins and catechins that can be found in the chemical composition.
Tannins and catechins are subcategories of polyphenols which provide anti-oxidant properties to oils. Astringent oils will also lessen the oily feeling (but not get rid of) of some oily products or ingredients. Anti-oxidants aid in extending the life of oils by delaying spoilage.
"Dry" carrier oils, such as calendula oil can be combined with heavier carrier oils such as olive oil to reduce the texture of heaviness and oiliness. Carrier oils are used to give massages that are sometimes mixed with essential oils. These oils are sometimes used to replace lotion or cream moisturizer for the skin and/or hair, some possessing medicinal properties for the skin. It is important to note that the vast majority of carrier oils have high amount of oleic acid that can break out acne prone skin if applied directly to the face or scalp. Try using carrier oils with low amounts of oleic acid to moisturize and nourish the hair and scalp such as grapeseed oil or jojoba oil if you have acne prone skin.
Essentials oils, also known as ethereal and volatile oils, provide many positive qualities for the hair, each with different properties. However, it is important to note that essentials should not be directly applied to skin and handled with care as the concentrations of these oils are very potent. Essentials should be diluted before application which can be accomplished by being mixed with carrier oils, conditioners, lotions, or shampoos. These oils are the essences of plants or leaves which is the reason for the potency and are not as heavy as carrier oils. Essential oils are light enough to evaporate, possess an aromatic fragrance, and are used for aromatherapy purposes as well.
Carrier and essential oils do not have a standard length of life before spoilage, therefore, check with the supplier before purchasing to verify the shelf life. Add Vitamin E Oil, an anti-oxidant, to extend the lives of oils. Also, keep the oils in a cool and dark location to extend the shelf life as well.
FYI: I was told to not pour oils in conditioners and shampoos that are already made and bottled. The oils and product will just separate. Instead, pour out a portion of conditioner and shampoo that is to be used with oil. Furthermore, the shampoo and conditioner have a different shelf life than oils thus the oils will have an impact upon when the hair product will go rancid. So, if an oil with a six month shelf life is poured into conditioners or shampoos then the life these hair products will decrease.
However, I do pour oils in my shampoos and conditioners. I don't know if the products separate c/ I just shake the shampoos and conditioners to mix everything again.