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Five types of Base Oil according to API (American Petroleum Institute)

Group Viscosity Index Saturates Sulphur in % Description
I 80-120 < 90% > 0.03% Conventional (Solvents)
II 80-120 ≥ 90% ≤ 0.03% Requires Hydroprocessing
III >120 ≥ 90% ≤ 0.03% Requires severe Hydroprocessing, often special feedstocks
IV     --- PolyAlphaOlefins (PAO)
V     --- All other basestocks not in Group I – IV including other synthetics

Note that the base oil group category is followed by the manufacturing method (in bold print) and then a description of the oil characteristics for each category. 

Group I - Solvent Freezing
Group 1 base oils are the least refined of all the groups. They are usually a mix of different hydrocarbon chains with little or no uniformity. While some automotive oils on the market use Group I stocks, they are generally used in less demanding applications. 

Group II - Hydro processing and Refining
Group II base oils are common in mineral based motor oils currently available on the market. They have fair to good performance in lubricating properties such as volatility, oxidative stability and flash/fire points. They have only fair performance in areas such as pour point, cold crank viscosity and extreme pressure wear. 

Group – III Hydro processing and Refining
Group III base oils are subjected to the highest level of mineral oil refining of the base oil groups. Although they are not chemically engineered, they offer good performance in a wide range of attributes as well as good molecular uniformity and stability. They are commonly mixed with additives and marketed as synthetic or semi-synthetic products. Group III base oils have become more common in America in the last decade. 

Group IV - Chemical Reactions
Group IV base oils are chemically engineered synthetic base stocks. Polyalphaolefins (PAO's) are a common example of a synthetic base stock. Synthetics, when combined with additives, offer excellent performance over a wide range of lubricating properties. They have very stable chemical compositions and highly uniform molecular chains. Group IV base oils are becoming more common in synthetic and synthetic-blend products for automotive and industrial applications. 

Group V - As Indicated
Group V base oils are used primarily in the creation of oil additives. Esters and polyolesters are both common Group V base oils used in the formulation of oil additives. Group V oils are generally not used as base oils themselves, but add beneficial properties to other base oils. Some examples of Group V Base Oils are: Alkylated Naphthalene, Esters, Poly-alkylene glycols, Silcones, Polybutenes.

Base Oils