Base Oil Groups I–V Explained for Lubricant Blending

The performance of any finished lubricant depends heavily on its base oil, which typically makes up 70-90% of the fluid. The American Petroleum Institute (API) categorizes base oils into five groups based on sulfur content, saturates, and viscosity index.

The API Base Oil Groups

• Group I: Solvent-refined. Lower saturates, higher sulfur. Used in older industrial applications and marine cylinder oils.
• Group II: Hydrotreated. Clearer, more stable, lower sulfur. The modern workhorse for most conventional motor oils and industrial fluids.
• Group III: Severely hydrocracked. Very high viscosity index. Legally classified as "synthetic" in many markets.
• Group IV: Polyalphaolefins (PAOs). True chemically synthesized base oils with exceptional temperature stability.
• Group V: All other base oils, including esters, PAGs, and silicones. Often used as additives or for highly specialized applications like aviation turbines.