Base Oil Basics
Most of the oils used in the automotive, commercial, industrial type sectors begin as a simple base oil so some understanding of what these base oils are is required to select the best product for the application. The American Petroleum Institute (API) categorize base oils into five categories (API 1509, Appendix E). The first three groups are base oils which are refined to different levels from petroleum crude oil. Group IV base oils are full synthetic (polyalphaolefin) oils. Group V is for all other base oils not included in Groups I through IV. Before all the additives are added to the mixture, lubricating oils begin as one or more of the following five groups.
Group I base oils are solvent-refined, which is a simpler refining process and also why they are the cheapest base oils on the market. Simply put, Group I oils contain the most amount of impurities and molecules are less uniform meaning lower performance than Group II and above oils.
Oils using a Group I base stock can often carry an OEM approval but do not offer the protection and superior performance of Group II or III oils. The use of a Group I base oil is easily seen in a AW 46 hydraulic oil where the colour is of a dark brown/honey colour which indicates lower quality or rerefined/recycled base stock. Group II AW 46 is clear and easily distinguishable.
Group II base oils are manufactured by hydrocracking, a more complex process than the Solvent Refining process used for Group I oils. Since all the hydrocarbon molecules of these oils are saturated, Group II base oils have substantially better antioxidation properties and offer longer service life. They have a clearer color and cost more in comparison to Group I base oils. Group II base oils are becoming very common on the market today and are priced very close to Group I oils while offering far greater performance.
The advantages of Group II over Group I are: Better thermal stability, increased oxidation stability, increased service life, greater component protection and better resistance to the formation of lacquers and sludge.
Group III base oils are refined even further than Group II base oils and are generally severely hydrocracked under higher heat and pressure to increase VI. Using a higher VI feedstock also produces a higher VI product. The longer process is designed to achieve a purer base oil. Although made from crude oil, Group III base oils are sometimes described as synthesized hydrocarbons. Like Group II base oils, these oils are also becoming more commonly used and price is decreasing due to demand.
Group IV base oils are polyalphaolefins (PAOs). These synthetic base oils are made through a process called synthesizing. They offer a much wider temperature range and are excellent for use in demanding conditions and extremely cold/hot applications.
Group V base oils are classified as all other base oils, including silicone, phosphate ester, polyalkylene glycol (PAG), polyolester, biolubes, etc and are used primarily as oil additives. These base oils are at times mixed with other base stocks to enhance the oil’s properties. An example would be a PAO-based compressor oil that is mixed with a polyolester.
Esters are common Group V base oils used in different lubricant formulations to improve the properties of the existing base oil. Ester oils can take more abuse at higher temperatures and will provide superior detergency compared to a PAO synthetic base oil, which in turn increases the life time of the oil.