Can the regular use of a pre-shave oil coat your shaving brush with a layer of oil that would potential damage it (e.g., clumping of badger hairs) or reduce its effectiveness?
When pre-shave oil is used properly, it would likely take quite a while before any negative effects on the shaving brush would be noticed. The most common mistake that wetshavers make with pre-shave oil is using too much of it for each shave. Pre-shave oil is best used in a very small amount, only enough needed to coat the very surface of the skin with a light amount of oil. Applying it in this manner prior to lathering with the brush would expose the bristles of the brush to a very small amount of the oil. However, if a heavy coat a pre-shave oil is applied prior to shaving, not only would it be more likely to gunk up the brush but it would also negate any of the lubricating and moisturizing properties of your favorite soap or cream. See also: Will shaving oil ruin a badger brush?