Understanding the differences between oils, lubricants, and greases is essential for engineers, maintenance teams, and procurement specialists who want to maximize equipment life, reduce downtime, and optimize total operating cost. Although the three terms are often used interchangeably in casual conversation, each has distinct chemistry, performance characteristics, and best-use scenarios. This guideline explains what each product class is, how they work, how they’re specified and tested, how to select the right product for an application, and practical maintenance and environmental considerations. The content is written for technical decision-makers and is optimized for search engines with practical, actionable detail.

1. High-level definitions

• Oil — A fluid lubricant whose primary component is a liquid base (mineral, synthetic, or bio-based). Oils flow freely at operating temperatures and form hydrodynamic or elastohydrodynamic films to separate moving surfaces.

• Lubricant — A broad umbrella term that includes oils, greases, pastes, and other materials whose purpose is to reduce friction, wear, corrosion and to carry away heat. In many contexts “lubricant” simply means the chosen product for a lubrication point.

• Grease — A semi-solid or solid-like lubricant made of a base oil entrapped in a thickening agent (thickener) and performance additives. Grease behaves like a solid at rest and releases oil when sheared, providing localized lubrication and staying in place where oils would run off.

2. Basic chemistry and constructionOils

• Base fluids: Mineral oils (refined petroleum), synthetics (PAO, ester, polyalkylene glycols), and bio-based oils (vegetable esters).

• Additives: Anti-wear (AW), extreme pressure (EP), antioxidants, detergents/dispersants (in engine oils), anti-foam agents, corrosion inhibitors, pour point depressants, viscosity index improvers.

• Function: Form a continuous fluid film between surfaces; heat transfer medium; carrier for additives and contamination.

Greases

• Thickener types: Soap-based (lithium, lithium complex, calcium, aluminum), non-soap (polyurea, silica, bentonite/clay).

• Base oil: Same kinds as oils — mineral or synthetic — selected for viscosity and temperature performance.

• Additives: Similar additive packages as oils, though concentrations and functions (e.g., anti-wear, oxidation inhibitors) are tuned for grease behavior.

• Structure: A 3D network of thickener fibers that traps oil. The “worked penetration” and NLGI grade define consistency.

3. How they lubricate: lubrication regimes

• Hydrodynamic lubrication — A full fluid film separates surfaces (common with oils in bearings, journal bearings). Requires sufficient speed and oil viscosity.

• Elastohydrodynamic lubrication (EHL) — High-pressure contacts (rolling elements, gears) where elastic deformation and lubricant viscosity elevate film strength.

• Boundary lubrication — Occurs at low speeds/high loads where additive films or solid coatings protect surfaces; greases frequently operate under boundary conditions.

• Mixed lubrication — Partial fluid film with intermittent asperity contact; common in transient conditions (startup/shutdown).

Greases can provide both fluid film action (by releasing oil) and boundary protection (thickener and additives). Oils primarily provide fluid film lubrication.

4. Key performance properties and tests

For oils

• Viscosity (kinematic, typically measured at 40°C and 100°C) — dictates film thickness and shear. Viscosity index (VI) indicates how viscosity changes with temperature.

• Pour point — lowest temperature at which oil flows.

• Flash point — safety parameter for ignition risk.

• Oxidation stability — determines service life; influenced by base oil and antioxidants.

• Additive performance — antiwear/EP behavior, corrosion protection, foaming tendency.

For greases

• NLGI Grade — consistency classification (000 very fluid — 6 very hard). NLGI 2 is common for bearing greases.

• Worked penetration — measure of hardness/consistency after standard working cycles.

• Dropping point — temperature at which grease becomes fluid (indicator of upper temp limit for some soap thickeners).

• Oil separation/bleed — tendency of base oil to separate from thickener.

• Mechanical stability (shear stability) — how grease consistency changes under mechanical working.

• Water washout/resistance — critical for wet or washdown environments.

(Manufacturers commonly reference standards such as ASTM and ISO for these tests; confirm specific tests with suppliers.)

5. Practical differences — where oils excel and where greases are preferred

When to choose oils

• High-speed bearings, gearboxes, turbines, and hydraulic systems where continuous oil film and heat removal are required.

• Systems with oil circulation and filtration capabilities (sump, reservoirs, oil coolers, breathers).

• Situations needing precise viscosity control and exchangeable filtration/conditioning (oil analysis).

Advantages of oils:

• Better heat dissipation.

• Easier contamination control (filtration, centrifugation).

• More predictable viscosity control and film formation for hydrodynamic lubrication.

When to choose greases

• Enclosed or inaccessible points where re-lubrication intervals are long or relubrication access is difficult (wheel bearings, slow-speed plain bearings).

• Points prone to leakage or run-off where oil would not stay in place.

• High or low-speed boundary-lubricated contacts, small bearings, and where water resistance and sealing are required.

Advantages of greases:

• Stay in place — provide long-lasting lubrication at the point.

• Provide sealing against contaminants and moisture.

• Simpler system design (no pump, reservoir or return lines).

Hybrid scenarios

• Many systems use oils for circulating lubrication at centralized points and greases for local bearings or housings. Selecting compatible base oils and additives reduces incompatibility issues.

6. Compatibility and mixing — a critical caution

• Mixing greases with different thickeners (e.g., lithium with calcium or polyurea) can cause loss of structure, reduced performance, and catastrophic failures. Always follow manufacturer guidance.

• Base oil compatibility: A grease’s base oil must be compatible with the system oil if cross-contamination is possible.

• Seal and elastomer compatibility: Some synthetic oils and additives swell or degrade seals; check elastomer compatibility charts.

• Additive interactions: Certain additive chemistries can neutralize or accelerate wear if mixed improperly.

Rule of thumb: When in doubt, purge and replace rather than mix incompatible lubricants.

7. Selection criteria — how to choose the right product

Consider these factors in priority order:

1. Operating conditions: temperature range, speed (DN factor for bearings), load, shock loading.

2. Environment: water/washdown exposure, dust/abrasion, chemical exposure.

3. Maintenance strategy: centralized lubrication, re-lubrication intervals, oil change capability.

4. Mounting and accessibility: ease of relubrication or replacement.

5. Contamination control: filtration capability, seals, breathers.

6. Regulatory or safety requirements: food-grade (NSF H1), biodegradable, fire-resistant (HFCs/HFDs), low smoke/toxicity.

7. Life-cycle cost: evaluate total cost of ownership (lubricant cost + downtime + maintenance) rather than unit price only.

8. OEM specifications and warranty: follow OEM or standards where mandated.

8. Lubrication management and predictive maintenance

• Oil analysis (lubricant condition monitoring) — evaluates viscosity, contamination (water, particles), acid number, additive depletion. A cornerstone of predictive maintenance for oil-lubricated systems.

• Grease condition monitoring — more challenging; often assessed by disassembly inspections, temperature/vibration trends, or sampling if accessible.

• Relubrication intervals — should be based on operating data and condition monitoring; avoid over-greasing (can cause overheating and seal damage) and under-greasing (increased wear).

• Centralized lubrication systems — improve consistency and reduce human error; require periodic checks for pump performance and line integrity.

• Contamination control — use breathers, filtration, desiccant breathers, sealed housings; maintain cleanliness during relubrication.

9. Storage, handling, and shelf life

• Storage: Keep lubricants in original containers, sealed, in a cool, dry area away from direct sunlight and strong oxidizers.

• Shelf life: Varies by product; oils and greases may degrade (oxidation, additive breakdown). Check manufacturer’s shelf-life recommendations.

• Handling: Avoid contamination when dispensing — use dedicated pumps, dispensers, and clean funnels. Label containers to prevent cross-use.

• First-in, first-out (FIFO): Implement stock rotation to ensure fresh product use.

10. Environmental, safety, and regulatory considerations

• Biodegradable/sustainability: Bio-based and biodegradable lubricants are increasingly required for sensitive environments (marine, forestry, certain industrial sites).

• Food-grade lubricants: NSF H1 and ISO 21469 certifications for incidental food contact.

• Fire resistance: For high-temperature or high-risk areas, consider fire-resistant fluids (synthetic esters, water-glycols, etc.), but note trade-offs in wear protection and environmental impact.

• Disposal and spill response: Used oils and greases are regulated wastes in many jurisdictions — follow local disposal and recycling protocols.

11. Practical case examples

• High-speed electric motor bearings — typically lubricated with oil (circulating or splash) where heat removal is important. Grease may be used if oil system isn’t feasible, but select a high-speed lithium-complex or polyurea grease with appropriate base oil viscosity.

• Conveyor or slow-rotation bearings in dusty environments — greases with good adhesion, water resistance, and dust exclusion (calcium sulfonate or lithium complex) are preferred.

• Gearboxes — gear oils with EP additives and appropriate viscosity (ISO VG grades) and base oil selection for temperature and load. Grease-lubricated gears exist but are niche.

• Food processing equipment — NSF H1 greases or oils with low toxicity and suitable additive packages.

12. Purchasing & procurement tips for specifiers

• Request technical data sheets (TDS) and certificates — ensure VI, viscosity, base oil type, and additive slate are disclosed.

• Ask for compatibility and sample testing — get small samples for on-site trial and wear analysis.

• Consider supplier support — vendor lubrication audits, training, and oil analysis programs add value.

• Negotiate life-cycle pricing — include services (top-ups, condition monitoring) in procurement evaluation.

• Confirm warranty and liability — ensure use of a particular lubricant does not void OEM warranty.

13. Quick selection checklist (summary)

• Identify the lubrication point and duty cycle.

• Determine speed (DN), load, and operating temperature.

• Decide if oil circulation & filtration are available.

• Choose base oil type (mineral vs synthetic) for the temp/load profile.

• Select grease thickener type only after confirming compatibility and environment.

• Confirm additive needs (EP/AW, corrosion inhibitors, detergency).

• Validate with lab tests or vendor trial and implement condition monitoring.

14. Conclusion

Oils, lubricants, and greases each have clear, distinct roles in modern machinery maintenance. Oils are indispensable wherever continuous fluid-film lubrication, heat transfer, and filtration are required. Greases are the practical choice for localized, long-lasting lubrication where containment and protection from contaminants or moisture are priorities. The term “lubricant” encompasses both and underscores the importance of choosing the right chemistry and delivery method for the application. Proper selection, handling, compatibility checks, and condition monitoring are the pillars of effective lubrication management and can dramatically reduce wear, energy use, and downtime.

For procurement and engineering teams: treat lubrication as a systems decision — match the product to the mechanical, thermal, and environmental realities of the equipment rather than purchasing by price alone.

 Contact Aleman Moil today to secure the right lubrication solutions for your business needs!