Lubricating oils and greases can be classified in many ways; by their end use, by viscosity grades, by their additives package, or by their producers’ brand names. The most popular classification of lubes is according to their usage:

• Engine oils (petrol and diesel engines, aircraft, marine engines)
•  Turbine oils
•  Gear oils
• Compressor (refrigeration, air) oils
• Quench oils used in metalworking
• Cutting oils (in metal cutting)
• Insulating oils used in transformer and circuit breakers
• Wire rope lubricants
• Chain lubricants
• Hydraulic oils

Classification according to viscosity has been done by these professional societies and organizations:
• SAE (Society of Automotive Engineers, USA)
• API (American Petroleum Institute)
• AGMA (American Gear Manufacturers Association)
• NLGI (National Lubricating Grease Institute)

For industrial lubricants, many different classification systems have been used in the past in different parts of the world. In 1972, a worldwide viscosity  classification system for industrial lubricants came into place. Table 11-1 shows the current International Standards Organizations (ISO) 3448 viscosity classification system. The classification is based on series of viscosity grades, each being approximately 50 percent more viscous than the preceding grade. Viscosity variation within the grade is plus or minus 10 percent of the nominal viscosity of the grade. For lubricating oils other than automotive oils, the ISO VG classification system is generally used. This classification is in terms of viscosity only; other characteristics are not considered. Each ISO viscosity grade number corresponds to the midpoint of the viscosity range expressed in centistokes (cSt) at 40°C. For example, a lubricant with an ISO grade viscosity of 100 has a viscosity in the range of 90 to 110, with 100 the midpoint of the range.

Lube oils may be classified by additive type as follows:
• Inhibited or RO (rust and oxidation inhibitor containing additives)
• Anti-wear (AW) containing lube oils
• Lubes with extreme pressure (EP) additives
• Compounded oils: containing 3 to 10 percent fatty or synthetic fatty oils
• Residual compounds: These are heavy-grade straight mineral oils with asphaltic component for
open gear application where tackiness is required to increase adhesion