Tech Stuff

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.

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