DMT recommends you use DMT sharpener products dry or with water only. Although mineral oil or WD40 will not harm the DMT stones, they can impede sharpening if used in excess and can damage the sharpener overtime if not properly cleaned/removed from the sharpener.
Oil or oil-based products can clog up the stone, creating a barrier between the stone and the edge that you are sharpening which is unnecessary and is usually messy. It can then be transferred to all of the edges you are sharpening. We do not recommend any honing or lapping fluids.
The only stone we recommend any addition to water is the 8 microns; extra-extra fine diamond (EE); a drop of dish soap can be added since metal tends to bind on this diamond surface.
For storage, we advise that you should always wipe your Diamond Sharpeners clean and store dry after use. Put away your benchstones dry and store in a dry area. Use leafing (paper towels, cloth, plastic storage bag) in between your bench stones to prevent abrasive sides from rubbing. Additional recommendations to preserve quality over time include using an air-tight case, or including moisture absorbing silica gel packs with the stone in storeage.