These are the chemical substances which are added to the lubricating oil either to reinforce some of its natural properties or to provide it with certain new properties which it does not possess originally.

Additives of lubrication oil and their function:

1) Viscosity index improvers:- long chain, high molecular weight polymers

2) Pour point depressants:- Alkyl aromatic polymers

3) Antioxidant:- Aromatic amine compounds

4) Extreme-pressure (E.P.) additives:- polysulfides, phosphate, dithiophosphates, and dithiocarbamates.

5) Oxidation inhibitors:-Organic compounds such as amines, sulphides or phenols with metals like tin, zinc

6) Anti-foaming additives: - polyorganosiloxanes.