You don't "need" either one. I used my diamond stones dry for years, then started using a small amount of mineral Oil. Just enough to keep the Dust from getting on everything. I was doing a LOT of sharpening at the time. I found I liked the feel it provided. It was easier to literally feel the edge alignment on the stone with a little oil. The amount of oil required for this is very small, however, YMMV. Use whichever method (dry, water, oil) makes it easier for you to sharpen consistently.