Looking into the owner’s manuals of modern motorcycles you will find callouts for engine-oil viscosity and an American Petroleum Institute (API) service classification. The viscosity will be a multigrade with a designation of the form 10w-40, and the API classification will consist of two letters, such as SN. This tells us the viscosity required by the engine, sometimes different between warmer and colder weather, and the set of performance tests that the oil must meet.

It is rare for synthetic oil only to be specified for a motorcycle. Any oil of the correct viscosity grade that meets the given API test set will do a good job in your engine whether its base oil originates from synthesis (joining many basic chemical units together to engineer complex oil molecules) or from petroleum precursors that are refined and chemically altered to create the desired molecules.

You are free to choose any synthetic or petroleum oil that meets the callout of your engine’s manufacturer. It is the result of intensive and long-term oil testing.

“Yes, but isn’t synthetic just better all around? Doesn’t that mean I ought to bite the bullet and pay whatever it costs to protect my engine best, even if I never race or cross Death Valley in August?”

Oil is not the only part of your engine that can be improved at higher cost to better withstand extreme conditions. Lightweight titanium connecting rods would reduce crank bearing loads, extending their life at high rpm—Yamaha's YZF-R1M has 'em! Order today at $2,000–$4,000 a set, plus installation. Okay, maybe that's a lot, but con-rod cap bolts are very highly stressed, so shouldn't I at least have my stock ones replaced with $32-apiece multi-phase ultra-duty bolts just in case?

It comes down to personal preference.

This issue used to be a no-brainer back in the days when petroleum/mineral oils were made by just refining. Refining means removing less desirable elements from petroleum until what’s left is mostly higher-performing molecular structures. Waxy stuff had to go because it made oil congeal at winter temperatures. Aromatics had to go because they lost viscosity too rapidly when hot. Unsaturates had to go because they were vulnerable to heat-driven gumming and sludging. And so on.

But today’s top petroleum oils are made in quite a different way, not by throwing out the bad but by altering existing molecular structures to become preferred alternatives. Through the development of processes whose actions are highly specific, undesirable structures such as aromatics or long, straight chains are converted into desirable structures such as branched chains of particular geometry.

Advantec Ultimate
For much of the past decade, BMW has recommended just two oil viscosities—5w-40 and 15w-50—for all its motorcycle engine types. Advantec Ultimate and Advantec Pro lubricants were developed with help from Shell.Courtesy of BMW

The result of this work has been a convergence in properties between the best synthetics and the best oils originating from petroleum. After all, both are hydrocarbons. At the end of the 1990s, Mobil asked that Castrol be required to stop calling its petroleum-originated engineered oil “synthetic.” Castrol argued that the term synthetic could perfectly well describe oil molecules whose shape had been fundamentally altered by chemical engineering. In a 1999 decision by the Better Business Bureau’s national advertising department, chemically engineered and reshaped petroleum-based oils can be called synthetic. This is because what is important is that, in both cases, desirable, higher-performing structures have been created, not just separated out of petroleum as cream is separated from milk.

Because of the proliferation of types of oils, in 1993 the API created its composition categories:

Group I: Petroleum oil containing 10–30 percent aromatics. This is engine oil pretty much as it was in the 1930s through the 1950s, the era of the 1,000-mile oil change. As great granddad used to say, "Gimme an oil that's 100 percent oil, none o' them additives."

Group II: Hydrotreated mineral oils. Hydrotreating removes compounds of oxygen, nitrogen, sulfur, chlorine, and metals.

Group III: Severely hydrocracked oils, the extensively reformed "synthesized hydrocarbon oils" of today. Hydrocracking converts heavy oils into lighter fractions by saturation, breaking large molecules into smaller ones and creating from the fragments desired isomers. Single-pass isomerization can boost the Viscosity Index (VI) of a Group III base stock by 30 percent. VI indicates the rate at which an oil's viscosity decreases as temperature increases in roughly the range between cold and hot engine temperatures. The higher the number, the better the oil covers the range of temperatures.

Group IV: The dominant automotive synthetic today is PAO (Polyalphaolefin) + AN (Alkylated Naphthalene) + diesters, as found in synthetic engine oils like Mobil 1. PAO delivers exceptional VI and low pour point, while its relative lack of lubricity and tendency to shrink seals are corrected by added AN and diesters. The PAO itself is synthesized by polymerizing simple gas molecules derived from natural gas, and then connecting these short hydrocarbon chains to form comblike structures.

Group V: Esters, polyglycols, silicones, neopentyl polyol esters, and others, which are synthetic or chemically engineered oils not falling into previous groups. Many are familiar to bike racers of the 1970s through 1990s because they have been the basis of two-stroke racing oils. Some of these oils can have higher VI, oxidation resistance, and resistance to thermal breakdown than do PAO-based synthetics. R&D never ceases in the chemical industry.

PAOs were first synthesized in the 1930s but were not produced. In 1951, Gulf developed a PAO production process. At first, such oils were considered useful mainly for their low pour point, which enabled diesel engines to be started in the arctic. In the 1960s, Mobil improved the Gulf process and marketed PAO-based oils in Europe, where small, hard-working auto engines created demanding conditions.

Shell replied with hydroisomerization to manufacture higher-performing mineral-based oils in Europe. This was basic economic competition. In 1993, Chevron began to reshape straight-chain oil molecules by adding side chains like those in PAOs.

All of this relates to base oils but does not discuss the additives that greatly broaden an oil’s capabilities. In one example of a finished lube oil’s composition, 78 percent was base stock and the other 22 percent were additives:

  1. Dispersant and detergent to prevent sludging or to isolate soot produced by direct fuel injection

  2. Antiwear agent to protect parts surfaces with a solid lubricant layer when oil films become incomplete—during cold starting, for example

  3. Inhibitor to protect against rust and corrosion

  4. Anti-foaming agent

  5. 10 percent VI improver to help combine low pour point with retention of useful viscosity at high temperature

Additive chemistry has been under development since the 1930s, but one aspect of additives seldom discussed is their ability to remain in solution in the base oil. Oil chemists note additive packages that work well in petroleum-originated oils do not automatically remain in solution when added to synthetic base stocks. Altered or specialized additives have been created to deal with this. Thus no one type of oil is automatically better in all respects. Good lubrication is a combination of base oil properties with additive performance. Oil chemistry has never stood still. It is constantly developing new processes and performance capabilities.