Motor Oil Base Stock Groups

I have heard motor oil base stocks refered to as Groups. What does this mean?

There are five goups of base stocks, with each generally catagorized as follows (AMSOIL Synthetic Oil base stock is Polyalphaolefins (PAOs) with a small amount of ester to help control seal swell):

See Charts Below

Base Oil Category	Sulfur (%)		Saturates (%)	Viscosity Index
Group I	>0.03	and/or	<90	80 to 120
Group II	<0.03	and	>90	80 to 120
Group III	<0.03	and	>90	>120
Group IV	All polyalphaolefins (PAOs)
Group V	All others not included in Groups I, II, III or IV (Esters)

Base Oil Category	Manufacturing Method	Oil Characteristics
Group I	Solvent Freezing	Group I base oils are the least refined of all of 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	Hydroprocessing 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	Hydroprocessing 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 oil products have become more common in America during the past decade.
Group IV	Chemical Reactions	Group IV base oils are chemically engineered synthetic base stocks. Polyalphaolefins (PAOs) 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 exhibit a wide variety of properties specific to each individual oil's formulation. Group V base oils are generally not used as base oils themselves, but add beneficial properties to other base oils.

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