Important Factors When Selecting Gear Oil
Gear oil and its functions?
Gear oil is a special lubricating oil designed to protect gears and gearboxes by reducing friction, wear, heat, rust, and damage during operation.
When gears rotate, their metal teeth come into contact under high pressure. Gear oil forms a protective film between the metal surfaces, reducing friction and wear and helping prevent damage.
Main jobs of gear oil
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- Reduces friction between gear teeth
- Reduces wear and extends gear life
- Protects under heavy loads using EP (Extreme Pressure) additives
- Carries away heat
- Prevents rust and corrosion
- Controls foam
- Helps keep the gearbox clean
- Keeps the gearbox clean – Helps reduce sludge and harmful deposits.
- Extends equipment life – Proper lubrication reduces damage and maintenance.
- Separates water – Good gear oils help water separate from the oil.
Favorite Grades of Gear Oil
For industrial gear oil, the most common grades are based on ISO VG (viscosity grade):
| Grade | Simple meaning | Typical use |
| ISO VG 68 | Light/thin | High-speed, lightly loaded gears |
| ISO VG 100 | Light-medium | Faster industrial gearboxes |
| ISO VG 150 | Medium | General industrial gearboxes |
| ISO VG 220 | Medium-thick | Very commonly used industrial gear oil |
| ISO VG 320 | Thick | Heavy-duty gearboxes |
| ISO VG 460 | Very thick | Slow-speed, heavy-load gears |
| ISO VG 680 | Extra thick | Very slow, heavily loaded gears |
The easiest way to remember it is:
68 → 100 → 150 → 220 → 320 → 460 → 680
Lower number = thinner oil
Higher number = thicker oil
For many industrial applications, ISO VG 150, 220, 320 and 460 are the key grades to know.
For automotive gear oils, you will instead commonly hear grades such as SAE 75W-90, 80W-90, 85W-140, etc. These SAE grades should not be directly compared with industrial ISO VG numbers.
Gear oil additives
For gear-oil research, additives are chemicals mixed with the base oil to give extra protection and performance.
Important Gear-oil Additives
| Additive | What it does | Main benefit |
| EP – Extreme Pressure | Protects gear teeth under very heavy pressure | Prevents scoring and seizure |
| Anti-wear (AW) | Creates a protective layer on metal | Reduces wear |
| Antioxidant | Slows down oil oxidation | Longer oil life |
| Rust inhibitor | Protects iron/steel from water and moisture | Prevents rust |
| Corrosion inhibitor | Protects metal surfaces | Reduces corrosion |
| Demulsifier | Helps oil separate from water | Easier water removal |
| Anti-foam | Reduces foam formation | Smooth lubrication |
| Metal deactivator | Protects copper and other sensitive metals | Reduces metal corrosion |
| Pour-point depressant | Helps oil remain fluid at low temperature | Better cold performance |
| Viscosity modifier | Helps control viscosity change with temperature | More stable lubrication |
Most important: EP additive
For conventional heavy-duty industrial gear oil, the EP additive system is particularly important.
A commonly used technology is sulfur-phosphorus EP chemistry.
When gear teeth experience very high pressure and temperature, the EP additives react at the metal surface and form a protective layer, helping prevent severe wear, scoring and seizure.
Think of it like:
Base oil = lubrication
Additive package = protection
Base oil + correct additives = finished gear oil
What to research for developing gear oil
If you’re studying an ISO VG 220 or 320 EP gear oil, for example, don’t focus only on the names of additives. Research the complete package: EP performance, anti-wear, oxidation stability, rust/corrosion protection, copper compatibility, demulsibility and foam control.
The additive dosage also cannot be chosen simply as “more additive = better.” EP chemistry, base oil, copper corrosion, seal compatibility and other additives must be balanced and validated through laboratory testing.
How to choose the best Gear Oil?
The best gear oil is not always the most expensive or thickest oil. It is the oil that correctly matches the gearbox, load, speed, temperature and OEM requirement.
Most gear oil properties.
| Check | What to look for | Example |
| 1. OEM recommendation | Manufacturer’s required oil | Always check manual first |
| 2. Viscosity grade | Correct oil thickness | ISO VG 150, 220, 320, 460 |
| 3. Gear type | Spur, helical, bevel, worm, etc. | Different gears may need different oils |
| 4. Load | Light, medium or heavy | Heavy load may require EP protection |
| 5. Speed | Fast or slow gearbox | Faster gears generally need lower viscosity than slower gears |
| 6. Temperature | Operating and ambient temperature | High/low temperatures may favor synthetic oil |
| 7. Additives | EP, anti-wear, antioxidant, anti-rust, anti-foam | Depends on application |
| 8. Water exposure | Good water separation | Important in wet environments |
| 9. Specification | Required industry/OEM standard | e.g., DIN 51517-3 CLP |
| 10. Oil type | Mineral or synthetic | Choose according to operating conditions |
Look for a gear oil that provides:
ISO VG 220 + required specification + good EP protection + oxidation stability + rust protection + water separation + foam control.
Mineral vs Synthetic
Mineral gear oil is usually suitable for normal operating conditions and is economical.
Synthetic gear oil can be better for very high/low temperatures, longer oil life, difficult operating conditions or where improved efficiency is important, provided it is compatible with the gearbox.
A useful rule to remember:
Right viscosity + right specification + right additives + right application = right gear oil.
