The lubricant's job is to keep the stone cutting, not to make the blade slide. As a stone abrades steel it produces fine metal particles — swarf — plus loosened abrasive grain. Dry, that debris packs into the stone's pores and welds onto its surface, a condition called glazing or loading. A glazed stone burnishes the bevel instead of cutting it, and sharpening slows to a crawl.

A film of honing oil or water suspends the swarf and floats it clear of the surface, so fresh abrasive stays exposed stroke after stroke. The fluid also reduces friction enough to limit heat at the edge and gives a consistent feel that makes angle control easier.

The visible evidence is the grey slurry that forms as you work — that is the lubricant doing its job, carrying steel away from the stone. Wipe it off as it accumulates and refresh the film. A stone kept properly lubricated cuts faster, stays flatter longer, and needs far less aggressive cleaning over its life than one used dry.

Match the fluid to the stone's construction:

• Oil stones — Arkansas (novaculite), aluminium oxide 'India' stones, and silicon carbide 'Crystolon' stones — are designed for honing oil: a light, refined mineral oil. Once a stone has been used with oil it should stay an oil stone; water will no longer wet its oil-impregnated surface, so the choice is effectively permanent.
• Water stones — Japanese-style synthetic and natural stones — use plain water, often with the stone soaked before use. Never oil a water stone; oil soaks into the binder and ruins its fast-cutting, slurry-forming action. A drop of rust-inhibiting additive in the water bath protects tools and stone racks.
• Diamond plates run with water, a water-detergent mix, or a purpose-made low-viscosity lapping fluid. They need only enough fluid to float swarf; oil works but makes cleanup harder.
• Ceramic stones are usually used dry or with water, and are cleaned with an abrasive eraser or scouring powder.

When a stone ships with a manufacturer's recommendation, follow it — binder chemistry varies, and the maker knows what the stone tolerates.

None of the three is a good substitute, each for a different reason.

Motor oil is far too viscous, and modern engine oils carry detergent and anti-wear additive packages formulated for engines, not stones. The heavy film rides the blade above the abrasive, slowing the cut, and the residue drives grit deep into the stone's pores where it is hard to flush out.

WD-40 and similar penetrating sprays are thin enough to work briefly, but they are largely volatile solvent: the film evaporates mid-session, leaving swarf to embed just as if the stone were dry. They also leave a varnish-like residue with repeated use.

Cooking oils — vegetable, olive, and similar — are the worst option. They are drying or semi-drying oils that oxidise and polymerise in the stone's pores, gumming it into a glazed, sticky surface that may need serious reconditioning to recover.

Proper honing oil is inexpensive and lasts years: a light-viscosity, highly refined mineral oil that stays liquid, never gums, and rinses swarf freely. If caught without it, a food-grade mineral oil (laxative-grade from a pharmacy) is the one safe household stand-in.

Light — much lighter than most people expect. Effective honing oils sit roughly in the 10–35 cP range at room temperature, comparable to a light machine or sewing-machine oil and far thinner than engine oil. Products are sold as low-viscosity 'fine' grades for finishing stones and slightly heavier bodies for coarse, open-pored stones, but all of them pour freely.

Viscosity matters because the oil has two jobs in tension. It must be thin enough to wet the stone instantly, penetrate the surface texture, and release swarf when wiped away — but with enough body to hold a continuous film under the pressure of the blade rather than squeezing out entirely. Too thick, and the blade hydroplanes over the abrasive while grit stays trapped in the heavy film; too thin (solvent-like), and the film evaporates or migrates before the stroke is finished.

In practice: coarse synthetic stones tolerate a slightly heavier oil that clings in their large pores, while fine Arkansas and finishing stones reward the lightest grades, which let the fine grain bite. If a stone feels like it has stopped cutting the moment oil is applied, the oil is too heavy.

Choose the right product and yes. The concern is real: whatever fluid is on the stone ends up in trace amounts on the blade, and kitchen knives go straight from sharpening to food contact.

Look for honing oils described as food-grade or pharmaceutical-grade white mineral oil — typically compliant with food-contact regulations such as US FDA 21 CFR 172.878 or equivalent listings in other jurisdictions. These are the same highly refined, colourless, odourless oils sold as cutting-board and butcher-block conditioners. They are chemically inert, tasteless, and safe in the incidental quantities involved.

Avoid sharpening kitchen cutlery with industrial honing fluids that carry solvent carriers, rust-inhibitor additives, or sulphurised extreme-pressure agents — these are formulated for tool-room work, not food contact. Cooking oils remain a bad idea for the stone itself, as they gum its surface.

Regardless of oil, the practical routine is the same: after sharpening, wash the knife in hot soapy water and dry it before it returns to the kitchen. That removes both oil film and abrasive swarf — the swarf being the thing you least want near food.

A glazed stone — surface shiny, grey, and barely cutting — can almost always be recovered:

1. Flush with fresh lubricant. Flood the stone with honing oil (or water for a water stone) and rub firmly with a stiff nylon brush or coarse cloth. The fluid lifts much of the embedded swarf; wipe the dirty slurry away and repeat until the fluid stays clean.
2. Deep-clean an oil stone that flushing alone will not fix: warm soapy water and a scrub, or a soak in mineral spirits for heavily loaded stones, dissolves years of hardened oil and steel. Dry the stone fully before returning it to service.
3. Re-expose fresh abrasive. If the surface is still smooth and dark after cleaning, the pores are worn as well as loaded. Lap the stone on wet-and-dry paper over glass, a flattening plate, or loose silicon carbide grit on a flat tile — this removes the glazed skin and restores both cut and flatness in one operation.

Prevention costs less than cure: use enough lubricant every session, wipe the slurry off before it dries, and store stones clean. A stone that glazes repeatedly despite good habits is being asked to hone too-hard steel or is overdue for flattening.

Enough to keep a visible wet film across the whole working surface for the entire session — and no more. On a bench stone that means a generous line of honing oil spread with a fingertip or the blade itself, refreshed the moment any area starts to look dry or the slurry thickens into a paste. Most sharpeners re-apply once or twice during a typical edge touch-up and more often during heavy regrinding, when swarf production is highest.

Signals to read as you work: a grey, mobile slurry means the film is doing its job; a dry patch that squeaks or drags means add oil now; slurry turning stiff and dark means wipe it away and re-oil rather than pushing through it — thick paste holds swarf against the stone, which is exactly what the oil is meant to prevent.

Water stones follow the same logic with water: keep the surface glistening, sprinkle or spray as it absorbs, and let the slurry build — on many water stones the slurry itself does fine polishing work. Diamond plates need only a few drops spread thin.

There is no penalty for slight excess beyond mess; the real error is thrift. A stone run half-dry glazes in minutes, and reconditioning it takes far longer than the oil saved.