Because mineral oils attack them. Many elastomers swell, soften and lose their sealing ability when exposed to mineral oil, and some plastics craze or become brittle. Silicone is chemically compatible with most rubbers and plastics, so it lubricates seals, weatherstrips, plastic gears and slides without degrading them. The main exception is silicone rubber itself, which can absorb silicone fluids, so check compatibility on that specific material.

No, not as a substitute for grease. Silicone fluids have poor load-carrying and anti-wear performance, and a loaded bearing, gear or sliding metal contact will wear rapidly on silicone alone. Use it where compatibility with rubber and plastic, cleanliness, water repellency or wide temperature range is the requirement. Where the requirement is load, use a lithium, lithium complex or extreme-pressure product instead.

Silicone has extremely low surface energy and spreads across surfaces readily. Even trace contamination causes paint to withdraw from the affected spot, producing craters known as fish-eyes, and it interferes with adhesives and sealants in the same way. It transfers via hands, gloves, rags and aerosol overspray and is very hard to eliminate once loose in a workshop, so segregate it from any painting, coating or bonding area.

On anything that depends on friction. That includes drive belts and pulleys, brake components, clutch surfaces, walking surfaces, ladder rungs, steps and handholds, and any grip surface. It also should not be used where the part will later be painted, printed or bonded. Beyond that, avoid it on heavily loaded metal contacts, where it will not carry the load, and check compatibility before use on silicone rubber.

Less than an oil, which is one reason it is preferred on tracks, slides and seals in dusty or domestic environments, but it is not a dry lubricant and a heavy application will still collect dirt. Apply a thin film and wipe away excess. Where the environment is genuinely dusty or abrasive, a dry film PTFE or graphite product will stay cleaner and is likely the better choice for the job.

Consistency and where each stays put. A spray or fluid penetrates into seals, tracks, hinges and tight spaces, spreads thinly and is easy to apply to areas that cannot be reached by hand. A grease is thick, remains where it is placed, and is used on O-rings, connectors and seals where a lasting film is needed. Both are silicone based and share the same compatibilities and cautions.

Only if it is specifically formulated and registered for the purpose. General workshop silicone sprays are not food-grade, and many contain carrier solvents and additives that are not permitted in food production. Where incidental food contact is possible, use a product carrying NSF H1 registration and follow the site's lubricant control procedures, keeping food-grade and general-purpose products in segregated storage to prevent the wrong can being picked up.