Lubrication science
Additive families: what is in a finished oil and what each one does
Quick answer
Detergents, dispersants, anti-wear, EP, antioxidants, inhibitors, VI improvers and more, in one table.
A finished oil
A finished lubricant is base oil plus an additive package. In an engine oil the additive package can be 5 to 30 percent. In many industrial oils it is only a few percent. Additives do jobs the base oil cannot do, and they get used up with service.
The families
| Additive | Job | Typical chemistry | Indicative level, % by weight |
|---|---|---|---|
| Detergent | Neutralises acids and keeps surfaces clean | Calcium or magnesium sulfonates, phenates, salicylates | 2 to 10 (engine oil) |
| Dispersant | Holds soot, sludge and dirt in suspension so the filter can catch it | Polyisobutylene succinimides | 2 to 10 (engine oil) |
| Anti-wear (AW) | Forms a thin protective film in boundary lubrication | Zinc dialkyldithiophosphate (ZDDP), phosphates | 0.5 to 2 |
| Extreme pressure (EP) | Reacts under shock and heat to stop welding and scuffing | Sulphur and phosphorus compounds | 1 to 6 (gear oil) |
| Antioxidant | Slows oxidation and thickening | Hindered phenols, aromatic amines | 0.2 to 1.5 |
| Corrosion and rust inhibitor | Coats metal and stops water attack | Amine compounds, sulfonates, benzotriazoles for copper | 0.1 to 0.6 |
| Viscosity index improver | Reduces thinning at high temperature | Olefin copolymers, polymethacrylates | 0.1 to several |
| Pour point depressant | Keeps oil flowing when cold by modifying wax crystals | Polymethacrylates | 0.1 to 1 |
| Friction modifier | Cuts friction and saves energy | Organic compounds, molybdenum compounds | 0.05 to 0.5 |
| Foam inhibitor | Breaks foam bubbles | Silicones, polyacrylates | A few parts per million to 0.1 |
| Demulsifier | Helps water separate from oil | Specialised polymers | Very small amounts |
Balance, not quantity
More additive is not better. Additives compete for the same metal surface, and a package is tuned as a whole. Adding an aftermarket additive, or mixing oils, can upset that balance and cause sludge or reduced protection.
Depletion shows in oil analysis
- Falling TBN: detergent reserve is being used up.
- Falling zinc and phosphorus: anti-wear additive is being used up.
- Rising TAN and viscosity: antioxidant is exhausted and the oil is oxidising.
- Foam or slow water separation: foam inhibitor or demulsifier is gone or masked by contamination.
Takeaway. Each additive does one job and is used up at its own rate. Read used-oil results as a check on which additive has run down.
Put it to work in the tool