Base Oil

Base Oil

What is Base Oil?

The mineral oil term is used to encompass lubricating base oil derived from crude oil. The American Petroleum Institute (API) designates several types of lubricant base oil:

Group I – Saturates < 90% and/or sulfur > 0.03%, and Society of Automotive Engineers (SAE) viscosity index (VI) of 80 to 120 Manufactured by solvent extraction, solvent or catalytic dewaxing, and hydro-finishing processes. Common Group I base oil is 150SN (solvent neutral), 500SN, and 150BS (bright stock)

Group II – Saturates > 90% and sulfur < 0.03%, and SAE viscosity index of 80 to 120 Manufactured by hydrocracking and solvent or catalytic the waxing process. Group II base oil has superior anti-oxidation properties since virtually all hydrocarbon molecules are saturated. It has a water-white color.

Group III – Saturates > 90%, sulfur < 0.03%, and SAE viscosity index over 120 Manufactured by special processes such as ISO hydromerization. Can be manufactured from base oil or slack wax from the waxing process.

Group IV – Polyalphaolefins (PAO)

Group V – All others not included above such as naphthenics, PAG, esters.

The lubricant industry commonly extends this group terminology to include:

Group I+ with a Viscosity Index of 103–108
Group II+ with a Viscosity Index of 113–119
Group III+ with a Viscosity Index of at least 140

Can also be classified into three categories depending on the prevailing compositions:


Lubricants for internal combustion engines contain additives to reduce oxidation and improve lubrication. The main constituent of the such lubricant product is called the base oil, base stock. While it is advantageous to have a high-grade base oil in a lubricant, proper selection of the lubricant additives is equally as important. Thus, some poorly selected formulation of PAO lubricant may not last as long as the more expensive formulation of Group III+ lubricant.

Base Oil Specification: