Short answer
Mild oil separation is normal; serious separation (>5–10%) indicates the grease is failing and should be replaced.
What is oil separation?
Grease is a colloidal structure where a thickener locks base oil in place. During storage or use, some base oil separates from the thickener matrix and migrates to the surface or container bottom — this is called oil separation.
Two main causes
1. Physical pressure (most common)
- Heavy stacking during storage;
- Vibration in transit;
- Long-term standing (gravity slowly deforms the thickener skeleton).
2. Temperature increase
Higher temperature lowers base oil viscosity, making it easier to seep out of the thickener.
How to judge severity
| Separation | Status | Recommendation |
|---|---|---|
| < 3% | Normal range | Use as is |
| 3–10% | Mild separation | Stir and reuse |
| > 10% | Significant | Replace |
Industry standards (ASTM D6184 / GB/T 392) typically use a "separation ratio" to evaluate.
How to reduce oil separation
- Storage: 5–30°C, no heavy stacking;
- Transit: avoid severe vibration;
- Choose proper NLGI consistency — harder grease (3, 4) separates less but flows worse.
When to definitely replace
- Separation > 10%;
- Darkened color or burnt smell;
- Contamination with debris, water, or metal particles.
Related reading
- 《NLGI Grade Selection》— choose the right consistency to minimize separation;
- 《Mineral vs Synthetic Grease》— synthetic oils have less separation tendency.
Conclusion
Oil separation is not always failure — what matters is the amount and whether other anomalies appear. Mild separation can be stirred and reused; severe separation means replacement.