Oil Tech: Base Stock

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Apr2012 27

While non-detergent oils are comprised of only oil base stock, most engine oils on the market today contains several components, but they can all be classified into one of two basic categories: the base stock, which I will discuss here, and the additive package, which will be discussed in other oil tech articles (see the link at the bottom of the page).

Oil base stocks are classified based on criteria outlined by API, the American Petroleum Institute, and fall into one of the five groups, numbered one through five. Every engine oil will fall into one of the five categories, with Group V being a catchall for everything not meeting the criteria for the other four groups. Groups I and II are mineral oils, your basic petroleum based oil, and are divided, roughly, based on the amount of processing. Group III is a highly-processed petroleum based oil, and is considered a synthetic oil throughout most of the world. Group IV is a fully synthetic oil that uses a PAO base stock. Group V oils have such a wide variety of base stocks, it cannot be classified either way, and, again, is a catchall for all oil base stocks not comprised of mineral-based oils or PAO oils.

Group I base stocks contain less than 90 percent saturated hydrocarbons, also called saturates, alkanes, or paraffins, less than 0.03 percent sulfur, and have a viscosity index (refer to tech article on viscosity) between 80 and 119, inclusive. Group I base stocks contain fractionally distilled petroleum that is processed with solvent extraction techniques to remove waxes and increase oxidation resistance. Group I base oils contain a significantly higher amount of impurities when compared with Group II oils, and are not used in modern conventional oils.

Group II base stocks contain over 90 percent saturated hydrocarbons, less than 0.03 percent sulfer, and have a viscosity index between 80 and 119, inclusive. Group II base oils are also created from fractionally distilled petroleum, much like Group I base stocks. The biggest difference between Group I and Group II base stocks is the type of processing performed. Group II oils are subjected to a variety of hydroprocessing techniques, including hydrotreating, hydroisomerization, hydrocracking, and hydrofinishing, all of which will be discussed further in future articles. These processes strip wax, remove impurities, further refine the base stock by reshaping the oil molecules. The result is a mineral based oil with fewer impurities and much higher quality than the outdated Group I base stocks.

Group III base stocks contain over 90 percent saturated hydrocarbons, less than 0.03 percent sulfer, and have a viscosity index over 119. The defining difference between a Group II and Group III base stock is the viscosity index, with Group III oils having an “unconvential” viscosity index, and, as a result, Group III base stock are sometimes called unconventional base oils (UCBO) or very high viscosity index base oils (VHVI). Group III base oils are processed similarly to Group II oils, with the main difference being either a more severe hydrocracking process or higher grade feed stock, resulting in the improved viscosity index. Modern Group III base stocks can match or exceed the performance of Group IV PAO oils, and can be labeled and marketed as synthetic oil in the United States, as well as most of the world, with the exception being Germany and Japan. The reasoning is that a Group III base stock is processed, and its chemical composition altered, so extensively that it no longer resembles a mineral oil, such as Group I or Group II.

Group IV base stocks are made up of polyalphaolefins (PAO). Beyond that, API does not set forth any standards for viscosity index. Group IV oils are what is traditionally referred to as synthetic. Polyapthaolefins are created by, no surprise, polymerizing and alpha-olefin. Without getting into too much organic chemistry, it basically results in an oil that has smaller molecules, rather than long hydrocarbon chains, as you would find in a traditional mineral oil. Group IV oils are generally more expensive than Group III oils simply due to the cost of manufacturing associated with PAO production. While Group III oils can now be created with similar performance characteristics to Group IV oils (more on that later), PAO oils do, generally, have a lower pour point, making them superior in colder conditions, although there are pour point depressant additives today that help close that gap.

Group V base stocks are all oil base stocks not classified under Groups I-IV. These are typically created using a polyol ester (POE) or other esters, polyalkylene glycol (PAG), but Group V oils could be any oil that is not created from distilled petroleum or PAO. These are not as common in engine oil applications, and POE/PAG oils are more commonly found as a compressor lubricant in refrigeration systems, such as your car’s air conditioning system. There are, however, some ester-based engine oils on the market today.

There is a lot of controversy as to what can and cannot be called synthetic. As was mentioned earlier, in most of the world, Group III base oils are allowed to be labeled and marketed as synthetic. Up until 1999, it was the general consensus that only Group IV oils were fully synthetic. That changed when the National Advertising Department of the BBB decided that Group III oils could be considered synthetic. So, whether or not the oil is a PAO synthetic, highly refined and processed mineral oil, or a combination of the two, it can be labeled as synthetic at your local parts store.

With today’s technology, Group III and Group IV oils are both very high quality engine lubricants that will stand up to the extreme conditions of your engine’s crankcase. The most important factor in determining which oil is right for your car is not the type of base stock used, but, rather, whether it has been approved by the manufacturer to meet the criteria of the oil specification for your specific engine.

For the complete series of Oil Tech posts, check out this link

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