Oil Mixing Guidelines
Guidelines for mixing lubricating oils including when mixing is acceptable, how to minimize risks, and procedures for oil changeover in industrial equipment.
While mixing different lubricating oils should be avoided when possible, it sometimes occurs during maintenance. Understanding compatibility helps minimize risks when mixing is unavoidable.
General Principles
When Mixing Is Generally Acceptable
- Same manufacturer, same product line, same viscosity
- Mineral oil with mineral oil
- PAO with mineral oil (common in synthetic blends)
- PAO with ester (common formulation approach)
When Mixing Should Be Avoided
- Different additive technologies (even same base oil)
- PAG with mineral oil or PAO
- Silicone with hydrocarbon-based oils
- Any oil with PFPE
When Mixing Is Prohibited
- Fire-resistant fluids in systems designed for them
- Food-grade oils (NSF H1 status may be void)
- OEM-specific fluids under warranty
Additive Considerations
Even when base oils are compatible, additives may not be:
- Zinc (ZDDP) additives: May be incompatible with some ashless additives
- EP additives: Different chemistries may interfere
- Corrosion inhibitors: Mixing may reduce effectiveness
- Detergents/dispersants: Different types may precipitate
Viscosity When Mixing
Mixing oils of different viscosity results in intermediate viscosity. To estimate:
- Equal parts of ISO VG 32 and ISO VG 68 yield approximately ISO VG 46
- Use manufacturer blending charts for precise calculations
- Verify final viscosity meets equipment requirements
Oil Changeover Procedure
When switching oil types:
For Compatible Oils
- Drain as completely as possible
- Fill with new oil
- Run equipment briefly
- Check oil level, top up if needed
- Sample after short service for analysis
For Incompatible or Unknown Compatibility
- Drain completely
- Flush with compatible flushing fluid
- Drain flush
- Fill with new oil
- Drain and refill after short break-in period
- Sample for analysis to confirm contamination removed
Monitoring After Oil Changes
After changing oils, monitor for:
- Unusual foam or air entrainment
- Filter plugging
- Varnish or deposit formation
- Abnormal wear in oil analysis
- Changes in operating temperature
Compatible Combinations
| Base Oil 1 | Base Oil 2 | Compatibility | Notes |
|---|---|---|---|
| Mineral (Group I/II) | Mineral (Group I/II) | Compatible | Same base type |
| Mineral | PAO (Group IV) | Compatible | Common blend |
| Mineral | Ester | Generally compatible | Check additive interaction |
| PAO | Ester | Compatible | Common synthetic blend |
| PAO | PAO | Compatible | Same base type |
| Group III | Mineral | Compatible | Similar chemistry |
| Group III | PAO | Compatible | Used together in formulations |
Incompatible Combinations
| Base Oil 1 | Base Oil 2 | Result | Risk Level |
|---|---|---|---|
| Mineral/PAO | PAG (water-soluble) | Separation, poor lubrication | High |
| Mineral/PAO | Silicone | Separation, foam | High |
| PAG (water-soluble) | PAG (oil-soluble) | Possible separation | Moderate |
| Phosphate ester | Mineral | Additive interference | Moderate |
| PFPE | Most others | Separation, incompatibility | High |
Notes
- - Base oil compatibility doesn't guarantee additive package compatibility
- - Mixing oils of different viscosity grades results in intermediate viscosity
- - When in doubt, drain and refill rather than top up with different oil
- - Oil analysis can detect mixing problems before equipment damage occurs
Sources
- Machinery Lubrication - Oil Compatibility
- ExxonMobil Lubrication Engineering Guide
- SKF Maintenance Products Handbook
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