Synthetic engine oils: Addressing questions, mysteries and myths

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Synthetic engine oils: Addressing questions, mysteries and myths


Weber is president of Virginia-based Write Stuff. He is an award-winning freelance automotive and technical writer and photographer with over two decades of journalism experience. He is an ASE-certified Master Automobile Technician, and has worked on automobiles, trucks and small engines. He is a member of the Society of Automotive Engineers (SAE) and numerous other automotive trade associations. He has worked as an auto service technician, a shop manager and a regional manager for an automotive service franchise operation.

While the use of synthetic engine oil has become rather commonplace in today’s market, there still are questions, mysteries and myths regarding synthetic engine oil. And curiosity is again piquing in many car owners’ minds since so many makes and models now require synthetic oil.

A chemist at Mobil isolated the first polyalphaolefin (PAO) molecule while researching another project. It became nothing but an interesting lab book note back in 1949.

In the 1970s, the OPEC oil embargo prompted motorists and oil companies to find ways to conserve resources. That’s when Mobil 15W-30 was introduced, reportedly providing up to a 5% increase in fuel economy.

What is the fundamental difference between conventional and synthetic motor oils? Conventional oils, refined from crude contain a plethora of molecules. This variety makes the oil a good lubricant, for the most part. However, this mixture of molecules all has different boiling points and some break down more readily than others. The remaining molecules may cause the oil to become too thick at low temperatures and too thin at higher temperatures. The oil essentially wears out.

Synthetics, on the other hand, have just a few molecules — just the best ones for lubrication. They resist high temperature breakdown and keep their lubrication ability. In addition, since they have no waxy molecules, synthetics stay thinner and freer flowing at lower temperatures while resisting sludge formation at higher temperatures.

Breaking it down

There are five designated base stocks: Groups I and II are mineral oils derived from crude. Group III is a highly refined product of crude and is often claimed to be synthetic. Group IV base stock is PAO. Group V is the various synthetic stocks other than PAO, which are made of esters (acids condensed in an alcohol).

Group I is the cheapest with a viscosity index between 80-120, according to CRP Automotive. Group II is used in most standard motor oils. Group III is used with PAOs for use in semi-synthetic oils.

Oil change intervals

Since synthetic oils don’t break down as quickly as conventional oils, can you extend oil change intervals?

Yes, and no.

Many automobile manufacturers are specifying synthetic oil for their engines. It is the factory fill. In fact, the stipulation for time and mileage intervals is disappearing from owners’ manuals. Instead, the motorist is encouraged to rely on the vehicle’s onboard information center to get an oil change. This is usually different for different driving cycles and styles.

General Motors’ system is based on a computer algorithm calculating various inputs to compute the need for an oil change.

The oil life monitor (OLM) counts engine revolutions as its primary input, but it also looks at ambient temperature, the temperature when the engine is started, hot soak and cold soak, coolant temperature and other stuff. OLMs are tailored to the engine and vehicle in which it is installed.

Hence, a salesman who clocks thousands of miles per month will see a longer oil change interval than a soccer mom whose engine sees repeated cold starts and short engine operation.

Synthetic oil use

Which makes/models require synthetic oil?

The list keeps growing and would take up too much space here. The owner’s manual is the best guide.

One reason carmakers require synthetics is that they can spec a much lower viscosity oil which helps increase fuel economy due to reduced drag slogging through a heavier oil. In fact, any engine that requires 0W-20 requires synthetic.

Today’s engines are being built with much tighter tolerance ranges and clearances than ever and there are many rolling parts that now replace previous parts that made direct “pressure wipe” contact. Think of roller-lifters as the beginning of the trend. This puts less demand on the oil, including shear.

Synthetics provide more protection over a longer period of time. This allows oil change intervals to increase from the traditional 3,000 or 7,500 miles to 10,000 and sometimes beyond.


Engine oils contain additive packages that include viscosity index improvers, detergents and dispersants, and anti-wear agents such as ZDDP (zinc dialkyldithiophosphate... although ZDDP levels have dropped appreciably among many brands), but the newest oils, such as GM’s dexos ( do not contain zinc. That issue would require another magazine feature to report.

According to Liqui Moly, performance engine oils contain up to 35% additives and conventional engine oils hamper their additives’ efficiency in modern engines.

It is also a customer convenience since oil change intervals may well be longer. It not only saves oil and its potential environmental impact from improperly disposed used oil, it conserves time with less frequent trips to the shop. Of course, saving money may not be a factor since synthetic oils often cost four times as much as conventional motor oils.

Making the switch

What are the advantages of synthetics for diesel engines?

The new generation of diesels are now equipped with particulate traps to snag soot and ash. Most of it consists of zinc and phosphorus — the main chemicals in ZDDP, an additive that has been reduced or eliminated from most engine oils except for oils formulated for high performance, vintage and race engines. According to CRP Automotive, most diesel vehicle manuals state that warranties become void if you don’t use their recommended engine oil.

Must special, synthetic oil filters be used with synthetic motor oil? Can filter change intervals be extended?

No, and no.

Any high quality oil filter is acceptable with synthetics as well as conventional oils. And as you know, a higher quality filter often does a better job of catching and trapping harmful solids.

You should never suggest extending the filter change interval. It is a false economy to save a couple dollars on such an important, and reasonably priced, component. Changing the oil without changing the filter is like taking a shower with your socks on.

Likewise, never suggest extending the oil change interval beyond that which is published in the owner’s manual. For one thing, it could void the new car warranty. And you would not be doing your customer any favors. What the owner chooses to do beyond the warranty period is his or her choice. We have seen suggestions that certain brands of oil can be left in the engine for extended periods. Amsoil states that “there are no ‘consequences’ of extending drain intervals in a mechanically sound engine. For a warranty to become void, it must be proven that the oil was the direct cause of the engine damage. Vehicle manufacturers are responsible for defects in parts and workmanship no matter what oil is used. Bottom line: If the oil didn’t cause the problem, the warranty cannot be voided.”

Are there problems associated with switching from traditional engine oil to synthetic?

There is still some confusion about changing from conventional motor oil to synthetic oil in vehicles, particularly in older models with high mileage. But there are no problems associated with switching from conventional engine oil to synthetic as long as the engine has been properly maintained with routine oil changes and is not full of sludge.

Some very old engines may have been built using sealing technology that is not up to today’s standards and may not be compatible with modern oil... two-piece and rope main seals for, instance. Older engines also have wider tolerance ranges and often larger clearances accompanied by normal wear. These engines would be candidates for sticking with conventional engine oil. Switching to a full synthetic in older-design engines can easily result in annoying rear main leaks.

Nevertheless, if your customer’s vehicle is supercharged, turbocharged, driven on the racetrack, used for towing, or operated in extreme temperatures, synthetic oil may provide extended engine life. Each of these harsh operating conditions demand more from the engine and motor oil, and synthetic oil can deliver the needed protection.

Finally, these words of wisdom from J.D. Power, “There is little doubt that synthetic oil offers superior engine protection under extreme operating conditions. However, many owners may not operate their vehicles in conditions that warrant the additional engine protection of synthetic motor oil. For most owners, petroleum-based motor oils are just fine.

Change the oil at the manufacturer’s recommended interval (found in the owner’s manual) and your vehicle will reward you with a long service life.”   ●

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