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'Synthetic' as the word relates to motor oil

Monday, November 2, 2015

By Michael Trueba, Jr.
Images courtesy MPT Industries

MPT Industries’ Michael Trueba, Jr. realizes that synthetic oil is a confusing hot-button topic, and knowledge of what is really contained in the quart-sized bottles labeled “Synthetic” is hard to come by. In this article, he defines synthetic as it pertains to contemporary motor oil and explains why the definition is so broad; outlines the various qualities of conventional, partial-synthetic, and full-synthetic motor oils; and recommends two widely accepted test procedures that are worth your consideration when shopping for motor oil.

So you just purchased a case of what you believe to be high-quality synthetic motor oil from your local auto parts store or online retailer. How much of it is really the high-quality synthetic you think you bought? Although the majority of the big-name manufacturers offer them, it is widely accepted among industry experts that oils labeled “synthetic” are not equal, and some are not fully synthetic.

Let’s start our analysis with some background on the current, loose definition of synthetic oil, which is, an oil containing base stock that was synthesized through an intended chemical reaction. Crucially, there are no federal guidelines for using the word synthetic in the sale and distribution of motor oil in the U.S. As a result, the current definition stems from a complaint brought before the Better Business Bureau‘s National Advertising Division (NAD) on March 1, 1999 by Mobil regarding Castrol Syntec motor oil.

At the time, Mobil 1 was comprised of polyalphaolefin (PAO) base stocks, chemically synthesized from ethylene. Its complaint was centered on the fact that Castrol Syntec — comprised of hydrocracked or wax isomerized petroleum base stocks, or highly refined crude oil — was labeled and marketed as synthetic. In brief, the NAD ruled that the performance characteristics of Castrol Syntec and Mobil 1 were remarkably similar, and Syntec’s base stock was in fact synthesized from crude oil because it had an unnatural molecular arrangement that was a product of an “intended chemical reaction.”

This has allowed a broad interpretation of what is synthetic oil that runs contrary to what many oil experts now often label “true synthetic”— which includes oils that contain PAO, polyalkylene glycol, and/or esters as base stocks. The decision has also made the term synthetic less useful in determining oil quality. Established guidelines amongst major players such as Mobil and Castrol are the most widely accepted standards, however, some manufacturers and marketers also view the NAD’s decision increasingly as an open door to use cheaper, lower quality petroleum base stocks in formulating their synthetic motor oils. To compound this problem, many manufacturers will now blend synthetic with non-synthetic base stocks, lowering the bar even more as to what can be called synthetic.

A basic primer on lubricant base stocks reveals the five API categories that cover all conventional and synthetic oils. They are indispensible tools for understanding what is in oil and which type of oil is best suited for certain applications.

API motor oil categories and what they mean

Group I — Solvent-refined crude oil. High wax and aromatic (organic matter) content. Used in low-grade conventional motor oils.

Group II — Hydrotreated crude oil refining process. Less wax and aromatic content. Used in the majority of conventional motor oils.

Group III — Wax isomerized or hydrocracked crude oil refining process. Group III base stocks are considered synthetic because their molecular structures are altered through an intended chemical reaction. Very low wax and aromatic content. Used in the majority of synthetic motor oils.

Group IV — Polyalphaolefin (PAO) base stocks are chemically synthesized from ethylene. Used in some synthetic motor oils.

Group V — All other chemically synthesized base stocks, including all esters and polyalkylene glycol (PAG). Used in the minority of synthetic motor oils.

Though they have similar performance characteristics, should you be concerned about the substitution of Group IV or V with Group III base stocks in your synthetic motor oils? Your application, maintenance schedule, and the targeted price point will determine the answer. 

Full synthetic motor oils manufactured by the major brands and sold by the leading automotive chain stores are comprised of primarily Group III base stocks. Although some may also contain small amounts of Group II base stocks, they are good-quality motor oils with American Petroleum Institute (API) approvals that you can confidently put in your crankcase knowing they meet the requirements of new-car manufacturers.

However, by no means are Group III synthetic motor oils of the highest quality or necessarily the best product for your application. Pricing and affordability will be determining factors, as well as driving environment and intended use of your vehicle. A Group III-based full synthetic motor oil on sale at the local mega-store may be the best value if your requirements are ordinary, such as city or highway driving, the occasional autocross, and back road driving. But if vehicle use and ambient conditions will be more demanding, including high-performance driving, high heat or extreme cold, frequent short trips, longer oil change intervals, etc., a motor oil with Group IV or V base stocks may be for you.

Generally, Group IV and V base stocks, such as polyalphaolefins, esters, or polyalkylene glycols, will have performance advantages over Group III base stocks regardless of how well refined they may be. These advantages include a lower pour point (temperature at which the oil becomes semi-solid), less volatility or lubricant evaporation due to temperature, and better shear protection (more resistant to physical breakdown). Most formulators of high-performance synthetic oil will blend a variety of Group IV and V base stocks, which can help build a type of synergy between them. They also are often blended with Group III base stocks to help control the price and aid with the solubility issues sometimes associated with certain types of Group IV and V base stocks.

How to pick the correct oil for your application

Regardless of the type, synthetic base stocks normally make up about 70-85% of the overall oil content. The rest includes viscosity modifiers, friction modifiers, anti-wear agents, anti-foaming agents, corrosion inhibitors, dispersants, and other anti-oxidants to keep it from degrading over time.

Unfortunately, manufacturers and marketers of synthetic motor oils can be less than forthcoming with information regarding the types of base stocks and additives used in their product formulas. As a result, the only way to determine their quality and effectiveness is through a series of American Society of Testing Materials (ASTM) tests. For example, the Noack volatility test (ASTM D5800) measures the evaporation loss at high temperature. Another favorite is the High Temperature/High Shear (HTHS) test (ASTM D5481), which measures the viscosity of motor oil under high temperature and shear conditions. 

These tests are reliable indicators of the type and the quality of base stocks used in any motor oil. Unlike more traditional tests, such as viscosity index and flash point, which can be altered through the use of viscosity modifiers and other additives (short term modifications that show their weaknesses under performance testing), the Noack volatility and HTHS tests show the true capability or weakness of the base stocks when it is most important: under stressful, high-heat conditions. Obtaining the information may be difficult, however, since companies often do not publish all relevant testing information on their websites — sometimes none at all — and most consumers consider the tests to be prohibitively expensive to perform independently.

If you feel Group IV or V synthetic base stocks are important for your application, it is recommended that you do some research prior to shopping. Check oil manufacturers’ websites or company information for test data. (See sidebar below to find out how to look for and analyze these tests.) Motor oils that are comprised of Group IV synthetic base stocks generally have very low evaporation loss numbers and high numbers for shear testing. If this information is not readily available, email or phone the manufacturers. If they will not offer test results, it may mean that it is not in their best interest to do so.

Analyzing motor oil test results

How does one identify and analyze test results when they may not be publicly available? It can be difficult, but keep in mind that companies often encourage the use of test methods in which their products perform best.

Amsoil, for example, blends a high percentage of Group IV base stocks into its top-tier motor oils, so it behooves the company to promote the Noack and HTHS tests because they favor motor oils that have high percentages of Group IV and V base stocks. The same goes for MPT Industries’ top-tier motor oils, which contain a mix of Group IV and V base stocks to attain the desired viscosity range without using viscosity modifiers. Motor oils with Group I, II, or III base stocks will not perform as well, and therefore Noack and HTHS tests are often not published for them.

Most motor oils sold in the auto parts stores are approved by the American Petroleum Institute (API), which means they must meet certain standards. In the case of the Noack test, the maximum percentage allowed for API approval is 15%. The lower the test number, the less lubricant will be lost due to evaporation. It is worth noting that evaporation loss decreases as the viscosity of the motor oil increases. For example, Noack test results will generally be much higher with a 0W20 motor oil than a 20W50 motor oil. The best way to determine how good or bad a given Noack number is would be to compare it with some other competing brands in the same viscosity range (the same is true for HTHS).

High Temperature/High Shear testing is also a difficult test method that exhibits how well motor oil performs under high heat and high shear conditions. Motor oils that maintain their viscosity for longer periods perform better in this test. (Viscosity is measured in centistokes.) Once again, higher viscosity motor oils will tend to naturally outperform those with lower viscosity. For example, a 0W20 will not maintain its intended viscosity as long as a 20W50.

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