Lubricating Greases vs. Oils: Decision Guide for Engineering
A lubricating grease is a mixture of base oil and thickener; a lubricating oil is a pure liquid. This structural difference governs application area, speed limit, and maintenance interval. This decision guide is based on standards DIN 51825 (lubricating greases), DIN 51519 (oil viscosity grades), and DIN 51517 (lubricating oils).
Lubricating Greases
A lubricating grease consists of three components: base oil (typically 60–95%), thickener (5–15%), and additives (up to 10%). The thickener is a three-dimensional network that holds the base oil. Under mechanical load (shear), the oil is released and forms the lubricating film.
Advantages of grease:
- Stays in place – no drain system required
- Sealing effect against contamination
- Long maintenance intervals possible (lifetime lubrication)
- Easy application (grease gun)
- Better corrosion protection
Disadvantages: Limited cooling effect, not suitable for very high speeds (dn-value limit), harder to analyze for condition monitoring.
Lubricating Oils
Lubricating oils are liquid lubricants classified by their kinematic viscosity according to ISO VG classes. They are available as mineral oils, semi-synthetic, and fully synthetic versions.
Advantages of oil:
- Better heat dissipation (circulation lubrication)
- Suitable for very high speeds
- Easy condition monitoring via oil analysis
- Better for large gearboxes with circulation systems
- Easier to change (drain and fill)
Disadvantages: Requires sealed housing, drain system, possible leakage, more complex lubrication systems.
Direct Comparison
| Criterion | Grease | Oil |
|---|---|---|
| Heat dissipation | Low | High (circulation) |
| Sealing requirement | Lower | Higher (sealed housing) |
| Speed suitability | Medium (dn limit) | High |
| Maintenance interval | Long (lifetime possible) | Regular oil changes |
| Condition monitoring | Difficult | Easy (oil analysis) |
| Cost (lubricant) | Medium | Lower |
| System complexity | Low | Higher (pump, filter) |
| Contamination protection | Good (sealing effect) | Requires seal |
Typical Application Areas
Grease – Preferred Applications
- Rolling bearings (most applications)
- Linear guides and roller carriages
- Couplings and flexible joints
- Sealed small gearboxes
- Outdoor and highly contaminated environments
Oil – Preferred Applications
- Large industrial gearboxes
- High-speed rolling bearings (dn > 300,000)
- Hydraulic systems
- Gear transmissions with high heat generation
- Applications with continuous oil analysis
NLGI Classes for Lubricating Greases
| NLGI Class | Consistency | Typical Application |
|---|---|---|
| 000–0 | Very fluid to semi-fluid | Centralized lubrication systems |
| 1 | Very soft | High-speed spindle bearings, gearboxes |
| 2 | Butter-like | Rolling bearings (standard) |
| 3 | Firm | Wheel bearings, high-load bearings |
| 4–6 | Hard / block-like | Special applications, high temperatures |
Practical Recommendations
4-Step Decision Process:
- Check manufacturer specifications: The machine or component manufacturer usually specifies the lubricant type.
- Assess operating conditions: Temperature, speed, load, environment – determine which lubricant type is better suited.
- Consider maintenance options: Are regular oil changes feasible? Or is long-term lubrication (grease) preferred?
- Evaluate system costs: Include lubricant cost, maintenance effort, and required infrastructure in the decision.
From design to enquiry: procurement notes
- Cost drivers: The lubricant price itself is rarely the main factor. Regreasing intervals, labour time, and disposal costs determine total cost. NLGI 2 lithium grease and ISO VG 220 gear oil are close in material price – the TCO difference comes from system overhead.
- Standard vs. special grades: For standard rolling bearings and industrial gearboxes, an off-the-shelf branded product is sufficient (e.g. lithium complex grease NLGI 2 or ISO VG 220 CL oil). Specialty greases (PTFE, polyurea, food-grade NSF H1) only pay off when conditions clearly demand them: linear guides, food processing, or continuous duty above 120 °C.
- What an enquiry should include: Bearing type and bore diameter, operating speed (dn value), ambient temperature (min./max.), maintenance cycle (interval and accessibility), whether the system is open or closed.
- TCO aspect: Oil circulation systems require filtration, cooling, and regular oil changes – this increases operating costs when only a few lubrication points are involved. Long-life greases (regreasing every 12–36 months) significantly reduce maintenance costs for hard-to-reach bearings.
- Advice and sample delivery: TEA supports lubricant selection and alignment with your maintenance concept – get in touch.
Frequently Asked Questions about Greases vs. Oils
No, not directly: grease residues contaminate oil circulation systems and require a full cleaning before switching. The reverse (oil to grease) is equally critical, as grease dilutes the oil. Both lubricant types require separate lubrication systems. Contact us with any questions about switching lubrication systems.
The NLGI class (National Lubricating Grease Institute) classifies the consistency (stiffness) of greases on a scale from 000 (very fluid) to 6 (very hard block-like). Most rolling bearings use NLGI 2, which has a butter-like consistency. NLGI 0-1 is used for centralized lubrication systems, NLGI 3 for wheel bearings.
ISO VG (Viscosity Grade) describes the kinematic viscosity of an oil at 40°C in mm²/s (cSt). Common classes range from ISO VG 10 (very thin) to ISO VG 1500 (very thick). Gear oils for worm gearboxes use ISO VG 220–460, rolling bearing oils ISO VG 32–150 depending on speed and temperature.
Mineral lubricants are obtained from crude oil and are the cost-effective standard. Synthetic lubricants (PAO, ester, polyglycol) are chemically synthesized and offer longer service life, wider temperature range (–50°C to +200°C), lower evaporation, and better aging stability. They cost more but often enable longer oil change intervals.
Different lubricating greases should generally not be mixed, as incompatible thickener systems can interact and cause the grease to liquefy or harden. For oils, most mineral oils of the same viscosity class are miscible, but always check the data sheets. Mineral and synthetic oils should not be mixed without verification.
About the Author
Head of Procurement · Technische Antriebselemente GmbH
Thomas Albrecht manages procurement at TEA and advises customers on selecting the right lubricants for bearings, gearboxes, and linear guides. His expertise in lubrication technology helps optimize maintenance intervals and reduce lifecycle costs.