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.

There seems to be a lot of interest here about the base oil composition of the German Castrol 0W-30 oil, so I picked up a quart (Gold – M05216) at the local AZ and ran a G.C (Gas Chromatograph) and FTIR to take a peek at the components.

This oil is based entirely on 1-decene type PAOs. Judging from the ratios of the C30, C40, and C50 groupings it appears to be a blend of mostly 4 cSt PAO with either 6 cSt or 8 cSt cuts.

There is no mineral base oil present (Group I, II, or III) other than a small amount consistent with additive carrier oil.

No esters or ANs are present on the G.C. and transesterification does not remove any peaks (which it would if esters were present). The FTIR also shows no significant C=0 absorbance from esters. If there is any ester in this oil it is either very tiny or very high molecular weight, neither of which makes technical sense.