Will shaving oil ruin a badger brush?
I recommend applying the shaving oil directly to your skin and allowing it to thoroughly soak in prior to applying your shaving soap or cream with your brush. Do not add oil directly into the brush bristles or mix it into your lather because this will cause oil to coat your badger bristles. Using it under the lather will allow the razor to glide more easily across the skin while avoiding damage to your brush. I don't use oil for every shave, but I use it this way and have never had any problems with my shaving brushes. Also see How do you use pre-shave oil?