This is such a great question, thank you for asking it! In general terms, oil based formulations penetrate the epidermis better, however, it really does depend on the formulation and the actual active ingredients that have been put into the formulation. The other thing that you must take into account is that people have very different ideas about what is meant by “absorption or penetration” into the skin. Most products (again depending on their formulation and choice of active ingredients, and the molecular size of those active ingredients) can only penetrate into the upper layers of the skin, namely the epidermis. It is however in the dermis (the deeper layers) where our skin makes collagen and elastin and hyularonic acid. So to be perfectly honest there is no easy answer to your question, as it depends on: 1) The function of that particular product (ie. what is it supposed to do and in what are of the skin); 2) The formulation (how has the product been created and with what ingredients) 3) The base of the product, i.e. is the base suitable and able to penetrate the area in which the product is supposed to perform its specific function?