Engine oil -- Conventional, synthetic or semi-synthetic blend? What's the correct engine oil for your customers' vehicles?

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Engine oil -- Conventional, synthetic or semi-synthetic blend? What's the correct engine oil for your customers' vehicles?

Pity the poor do-it-yourselfer who decides to purchase engine oil at an auto parts store or big-box retailer. There are products making all kinds of claims for all types of vehicles — heavy-duty pickups and SUVs; high-performance, high-revving engines; high-mileage older cars; and so on. What should you suggest? Of course, the owner’s manual is the place to start.

The standard new-car oil is usually a premium conventional (mineral) oil. Service level SM is the current designation and is available in a variety of viscosity grades. Most carmakers are now suggesting 5W-20 or 5W-30. Conventional motor oil works very well and with regular changes there is no reason that an engine won’t achieve 200,000 miles.

Many carmakers are now calling for full synthetic oil in their high-tech engines. Corvettes and most Mercedes-Benz vehicles come with it as a factory fill. Synthetic oils pass stringent tests and offer longer-lasting performance when it comes to viscosity index and protection against deposits. Synthetics flow better at cold temperatures and maintain excellent lubricity at high temperatures. They do not readily evaporate. However, along with being more expensive, not every engine needs them.

Synthetic blends offer protection for somewhat heavier loads and higher temperatures. They don’t evaporate as readily as conventional oils. They are a good choice for drivers who put heavy loads on the engine such as towing or off-roading. Semi-synthetics offer many of the same benefits of full synthetics, but at a fraction of the cost.

People are keeping their vehicles much longer, especially in this weak economy. As you know, it’s not uncommon to see odometers well into the six-figure realm in your service bay. Heat and time can cause seals, such as main bearing seals, to harden and crack. Oil blenders formulate products with seal conditioners that get into the pores of the seals, helping them stay flexible and maintain their shape. To compensate for engine wear, high-mileage oils may be near the upper range of a given viscosity designation — a thicker 30-weight than another bottle may be. They generally have more viscosity improvers and anti-wear additives, too.

GM dexos specification

As a sidenote, the General Motors powertrain engineers have developed the dexos™ (lower case “d”) engine oil specification. The result is engine oil designed specifically for GM engines. And just like GM, dexos is global. The dexos spec is designed to increase fuel efficiency, extend the life of the emissions system, require fewer oil changes, and produce fewer emissions.

The dexos oil also has some unique properties that General Motors engineers required. One characteristic is better resistance to aeration (the whipping of air bubbles into the oil). Some engines with variable camshaft timing also force the engine’s oil supply to serve double-duty as a hydraulic fluid. If the oil becomes aerated the hydraulic actuators won’t operate properly (similar to how silicone brake fluid can aerate in an anti-lock braking system). GM requires dexos oils to better resist aeration.

Like dexos, the new GF-5 specification developed by the International Lubricants Standardization and Approval Committee (ILSAC) with input from automakers, oil refiners, and oil additive manufacturers is expected to offer better performance. The new GF-5 and GM dexos are backwards compatible; they will work in engines that have used older previous formulations.

The goal is to extend the time and mileage between oil changes. Both dexos and GF-5 oils allow more miles between oil changes without a loss of lubrication qualities, sludge buildup, or damage to the catalytic converter. Depending on your customers’ driving habits, oil changes could extend beyond 10,000 miles.

What’s the correct engine oil for your customers’ vehicles?

You have seen the starburst symbol and American Petroleum Institute (API) donut on oil bottles so often, you may forget that they are there.

API’s Engine Oil Licensing and Certification System is a voluntary licensing and certification program that authorizes engine oil marketers who meet specified requirements to use the API Engine Oil Quality Marks. Some brands of oil do not have the API insignia and some marketers claim that they are smaller companies that can’t afford to submit each of their products for approval. We will not make any judgments here.

The API program is a cooperative effort between the oil industry and vehicle and engine manufacturers. Performance requirements, test methods, and limits are cooperatively established by vehicle and engine manufacturers, technical societies such as the Society of Automotive Engineers (SAE) and the American Society for Testing and Materials (ASTM) and industry associations like the American Chemistry Council and API. Oils meeting these requirements are recommended by vehicle manufacturers, says the API Web site.


The API donut on the right tells you if the oil meets the current service rating which is currently SN for gasoline engines and CJ-4 for diesel engines. The most recent ILSAC (International Lubricant Standardization and Approval Committee) rating is GL-5. The donut also shows the SAE (Society of Automotive Engineers) viscosity number and whether the oil has passed the energy conserving test.

When suggesting oil to your customers, it is important to understand what the terms mean as well as what the various additives do. As you know, it is the additive package that wears out. The base oil can be re-refined and reused once a fresh additive package is introduced.

Viscosity is the oil’s resistance to flow. It is measured at 0 degrees Fahrenheit and the assigned number is followed by a “W” which stands for winter. As the oil heats up, its viscosity decreases, but the second number (determined at 212 degrees F) correlates to the amount of protection that a straight weight oil of that number provides. Hence, 5W-30 oil pours like a 5-weight oil yet protects like a 30-weight oil when at normal engine temperature. A more viscous oil seals the compression rings better and maintains a better film for bearings and other moving parts.

The lower the first number, the more easily the oil flows and gets pumped to critical places faster. And since the crankshaft sloshes through the oil in the pan, a lighter oil makes for easier starting after cold soak. It also improves fuel economy over a thicker oil. Mineral oils are able to achieve a 5W rating, but some synthetic oils meet the 0W specification.

In general, your customer should use the oil recommended in his owner’s manual for best overall satisfaction.

Resistance to thinning as temperature climbs is called viscosity index. Motor oil tends to lose viscosity from shear. Shear stability is vital to maintain a lubricating film between close-tolerance, moving parts.

The base oil package in the bottle may be anywhere between 70% to 95%. The rest is additives. Base oils differ in both composition and cost. Ditto for the additives. So the blenders make choices when formulating their products. Oil companies formulate their products to meet the requirements of the engine manufacturers — the automobile companies.

Viscosity-index additives reduce the tendency to thin at higher temperatures.

Detergents keep internal engine surfaces clean by preventing deposits from forming at high temperatures. They also inhibit rust and corrosion.

Dispersants keep solid particles in suspension to reduce the likelihood of forming sludge, varnish or acids.

Under some conditions, the oil’s film can break down. If metal parts contact one another, galling and damage occurs. To prevent metal-to-metal contact, anti-wear agents are added. The most popular and reliable has been a zinc and phosphorus compound called ZDDP (zinc diakyl-dithiophosphate). However, there is evidence that phosphorus tends to damage the catalyst in converters, and for that reason it’s being phased out.

Editor’s Note: Because ZDDP has been reduced in “mass market” engine oils, this leads to camshaft and lifter failures in older flat-tappet engines, and in some cases, also creates premature wear for rockers in engines that feature high valve spring pressures. As a result, special high-zinc oils and ZDDP additives have become readily available by specialty companies in the performance aftermarket. If an older vehicle equipped with a flat-tappet camshaft (or any modified engine with higher-than-stock spring pressures) enters your shop for an oil change, it is critical to use an engine oil that contains sufficient levels of ZDDP (or to add ZDDP concentrate to a conventional oil). ZDDP is absolutely critical for break-in of a new flat-tappet camshaft. However, even with a seasoned engine that features a flat-tappet cam, proper levels of ZDDP are needed in order to maintain high-pressure lubrication between the cam lobes and lifters. A high ZDDP level is also beneficial for many roller camshaft applications as well, specifically to protect lifter bodies and lifter bores, to prevent galling or scuffing issues where roller lifters exist in a tight oil clearance environment. ZDDP is also beneficial for roller rocker arms, especially where high valve spring pressures are present, to protect valve tips and pushrod tips. While the flat tappet issue served to bring the ZDDP content of oil (or lack thereof) to the forefront, a high level of ZDDP may be beneficial to any performance engine application where pressure and scuffing concerns are present.


Friction modifiers, such as graphite or molybdenum, reduce internal friction. As such, they improve fuel economy.

Pour-point depressants help the oil flow freely at low ambient temperatures, mainly by keeping wax particles in the oil from congealing.

Antioxidants, as the word implies, control the oxidation of the oil. As it oxidizes, it thickens. Oxidation (actually the breakdown of antioxidants) is one of the leading reasons oil should be changed.

Foam inhibitors help break bubbles in the oil. Aerated oil does not lubricate and since air can be compressed, oil may not be pumped adequately to all parts of the engine. On those occasions where the crankshaft churns through the oil in the pan (in the case of over-filling by an ill-informed customer), this “whipping” action results in foaming.

Rust and corrosion inhibitors protect components from acids and moisture. An oil’s total base number (TBN) is a measure of how well it can neutralize acids that form. (Here, the word base is the chemistry akin to alkaline.)

 API engine oil service categories

The current and previous API Service Categories are summarized in convenient charts. Refer to the vehicle’s owner’s manual before consulting these charts. Oils may have more than one performance level. For automotive gasoline engines, the latest engine oil service category includes the performance properties of each earlier category. If an automotive owner’s manual calls for an API SJ or SL oil, an API SM oil will provide full protection. For diesel engines, the latest category usually ­— but not always — includes the performance properties of an earlier category.

Service Category API CJ-4 describes oils for use in high-speed four-stroke cycle diesel engines designed to meet 2007 model year on-highway exhaust emission standards as well as for previous model years. These oils are especially effective at sustaining emission control system durability where particulate filters and other advanced after-treatment systems are used. For more information on API CJ-4 including frequently asked questions, please visit

Regarding full-synthetic oil for older engines

Even though a full-synthetic oil may indeed have performance enhancements (reduced frictional properties and higher thermal capabilities) as compared to a “conventional” petroleum oil, there are potential downsides to indiscriminant use of a full synthetic oil.

For an engine that has recently been rebuilt, the use of a full-synthetic oil may prove too “slippery” to allow proper piston ring seating, which can result in excessive blow-by. Granted, several late-model OEM engines use a full synthetic from the start, but their rings, cylinder wall surface finishes and cylinder bore geometry stability have been addressed to permit ring sealing with a full-synthetic oil.

If an engine has been rebuilt, especially an older engine design, it’s best to “break the engine in” using a petroleum oil in order to permit proper ring seating and sealing. Once the engine has been “seasoned” for at least a few hundred miles or so, a full synthetic oil may then be considered.

Another issue to be aware of deals with engine oil seals. Older engine designs (to generalize, let’s say older than the early 1980s) may utilize front and rear main seals that were not intended to run extremely thin (hygroscopic) oils. As a result, it’s not uncommon for a “vintage” engine to experience oil leaks when the owner switches to a full synthetic oil (if the engine leaked a bit before, switching to a full-synthetic oil will likely make the leak worse; and even if the engine previously did not leak, it may begin to leak with the use of a full-synthetic oil). We’re not implying that an older engine will experience external leaks, but the use of a full synthetic in these applications can increase the risk of such leaks. It’s something to be aware of if/when you service a customer’s older vehicle and/or vintage collector car. Full-synthetics are wonderful, but they have their place. It’s not necessarily the best choice for all engines.

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