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 are 150SN (solvent neutral), 500SN, and 150BS (brightstock)
• Group II – Saturates > 90% and sulfur < 0.03%, and SAE viscosity index of 80 to 120
Manufactured by hydrocracking and solvent or catalytic dewaxing processes. Group II base oil has superior anti-oxidation properties since virtually all hydrocarbon molecules are saturated. It has water-white color.
• Group III – Saturates > 90%, sulfur < 0.03%, and SAE viscosity index over 120
Manufactured by special processes such as isohydromerization. Can be manufactured from base oil or slax wax from dewaxing 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 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 more expensive formulation of Group III+ lubricant.