Maintenance Oils and Greases: Which One for Which Job

An air tool motor dies after three months. A fan bearing squeals then seizes. A door lock gets stiffer every week after someone sprayed oil into it. In all three cases the part was not faulty - somebody used the wrong lubricant while believing they were maintaining it.

Lubricants are not one liquid under different names. Each is designed for a job, and used outside it accelerates wear rather than preventing it. Manufacturer instructions are always the final word.

Oil versus grease

Grease is oil held in a thickener structure that keeps it in place and releases it gradually. That single difference explains everything else.

In practice: hinges, chains, fast bearings and enclosed machinery with an oil path want oil. Slow exposed bearings, joints exposed to water and dust, and points you cannot reach regularly want grease.

Penetrating oil is a freeing tool, not a lubricant

Penetrating oil has very low viscosity and surface tension so it creeps by capillary action into the microscopic gap between thread and fastener where rust forms. The common mistake is to spray and immediately apply full force, snapping the head and turning a five-minute job into two hours of extraction. Penetrant needs time, not more torque.

Clean surface dirt away, apply generously and wait at least ten minutes - hours or overnight in difficult cases, reapplying. Tap the head lightly, since vibration breaks the rust bond. Try tightening slightly before loosening, and use a wrench that surrounds the head rather than an open-ended or adjustable spanner. Moderate heat helps, but never a flame near flammable liquid, a fuel tank or unknown material.

Penetrant is not a lasting lubricant - most of it evaporates. If the part needs ongoing lubrication after reassembly, clean it and apply the proper product.

The key warning: general spray is not a bearing lubricant

The multi-purpose aerosol is genuinely useful for what it was designed to do: displacing moisture, freeing light seizures, cleaning residue, short-term rust protection. But most of these products are a water-displacing blend with a light penetrant and very little oil. The light fraction flashes off, leaving a weak film that carries no load, does not survive heat, and forms no protective layer between loaded moving surfaces.

Warning: using it as a permanent lubricant for bearings, gears or loaded shafts causes early failure, and it can wash the original grease out of a sealed bearing and leave it dry, destroying it within days. Use proper bearing grease or the specified oil.

Dry lubricants: the forgotten answer in dusty conditions

This is a frequent and damaging mistake locally. Oil or grease on an exposed surface in a dusty environment behaves like adhesive: it collects dust, sand and filings and within weeks becomes an abrasive paste between the moving surfaces - the lubrication itself turns into a grinding compound. Dry lubricants spray in a carrier that evaporates, leaving a thin slippery solid film that reduces friction without being sticky:

PTFE suits drawers, latches and plastic-on-metal mechanisms. Graphite is traditional for locks and keys and tolerates high temperature, though some modern locks specify particular products. Molybdenum disulphide handles high loads and extreme pressure.

Typical places: locks, latches, drawer runners, exposed mechanisms on site, and parts near saws where sawdust sticks to oil.

Grease compatibility and over-greasing

Grease is classified by thickener type (lithium, calcium, polyurea, clay) and base oil. Mixing incompatible types can break down the thickener in both, so the mix either runs out of the housing or hardens and stops lubricating - a bearing with no protection despite looking full. Stick to one type per machine, clean out the old grease when changing type, and never judge type by colour, which is only a dye.

More grease is not more care. Excess inside a sealed bearing raises drag and temperature, and the pressure can push seals out of position or split them, after which water and dust enter freely and the bearing fails. Follow the stated quantity and interval; where none exist, small and regular beats large and occasional.

Matching lubricant to application

Cutting fluid is a different thing entirely: during cutting, turning and drilling it cools the edge, reduces friction and flushes chips. Left on the work afterwards it can cause problems, since some water-miscible types encourage rust once dried. Clean, dry and protect after work.

Special considerations

• Food contact: equipment in kitchens, food plants or packing lines needs a lubricant approved for food environments; ordinary industrial product is not acceptable.
• Electrical contacts: do not spray general lubricant into panels, breakers or connectors - some products attack plastics and insulation, collect dust and alter contact behaviour. Use products made for the purpose, with the supply isolated.
• Rubber and plastics: some mineral oils swell or harden rubber, so check compatibility before lubricating seals. Every lubricant also has a temperature range; an ordinary one near an exhaust can carbonise into a hard deposit.
• Storage and disposal: keep containers closed, labelled and out of heat, keep the safety data sheet, and never pour used oil on the ground or into drains.

Summary

Oil flows, penetrates and cools but needs reapplying; grease stays, seals and resists contamination. Penetrating oil is a freeing tool that needs time, not force. General purpose spray is a water displacer, not a bearing lubricant, and it can wash a bearing grease out. In dusty conditions use a dry PTFE or graphite lubricant rather than oil that turns dust into grinding paste. Do not mix incompatible greases or over-grease, since excess destroys seals. Match the product to the job, remember cutting fluid is something else, and follow the manufacturer instructions in every case.