Preshave oil works to reduce nicks and irritation by increasing the skin suppleness and pliability NOT by providing lubrication
The bottom line: Well-moisturized and supple skin does not cut or become irritated as easily as dry skin.
A pre-shave oil that PENETRATES the skin holds water in the skin, and the skin is better moisturized and very supple. When a blade drags across the well moisturized skin, the skin "gives" to the blade and will bend or conform significantly to the shape of the blade BEFORE it will be torn or cut by the blade. This is the property of being supple. On the other hand, if the skin is dry, it will not be very supple. When the blade drags across the skin, the skin will not give or conform to the blade but will instead instantly tear and/or become irritated.
To further illustrate this consider your hands in the winter time. The skin is dry and so your fingers crack and cut easily because the skin is not supple. There are no cuts in the summer when the skin is well moisturized because the skin can bend with movement without breaking. Also, it is easier to break apart a dry carrot vs. a wet carrot or a dry branch vs. a wet branch etc. etc. A wet branch will bend SIGNIFICANTLY before it finally breaks, if it even breaks at all, whereas a dry branch will break instantly without bending.
This is why pre-shave oils work. Not so much because they add lubrication under the blade but more because they hold moisture in the skin allowing it to be as supple as possible under the blade. This is also why many report a dramatic decrease in nicks after using a pre-shave oil. Many also report softer skin after use because of this. Therefore, the best pre-shave oils are those that actually penetrate the skin and make it supple instead of oils that do not penetrate the skin. Oils that do not penetrate the skin would simply act to lubricate so the blade slides easier across the surface but would do nothing to increase the suppleness of the skin to reduce cuts. For a wet shave lubrication is mainly provided by water and lather from a soap or cream to decrease drag and blade "traction" into skin. The lather also adds a protective barrier between the skin and blade to keep the blade from touching the skin at all. Even if the blade does manage to contact the skin through a barrier and the skin is very supple it still won't cut easily.
In summary there are 3 strategic areas of attack to reduce shaving cuts and irritation:
- Physical barrier between skin and blade (cushion): Lather OR non-absorbable pre-shave oil
- Decreasing friction and blade traction (lubrication): WATER (most important) OR lather OR non-absorbable pre-shave oil
- Increasing skin suppleness (moisturizing): WATER (again most important) + absorbable pre-shave oil
Notice above how important water is (this is called WET shaving). This is a quote from an article in Harvard Health Letters on keeping skin supple as published in The Denver Post: "While the stratum corneum absorbs water nicely, it doesn't bind it very well, so some oily substance is needed to hold it in. Applying an oily substance to the skin without also resupplying it with water--either from the moisturizer or from another outside source like a bath--is ineffective" . Pre-shave oil holds the water in the skin to make it supple, however, you MUST also supply your skin with water at the same time so the oil can hold it in or else the pre-shave oil will be innefective. Therefore, I find that applying a good pre-shave oil in the shower works best. The water supplies hydration for the oil and the steam opens up your pores and allows the oil to penetrate better. You could also achieve the same results by applying it under a hot towel. I have found Jojoba oil to work as good as anything as it is similar in structure to human sebum and readily penetrates the stratum corneum. By the time you get out of the shower and are ready to shave your skin is nice and supple and maximally resistant to cutting from the blade.