If you ever want to start a lively argument among motorcycle owners just ask the question of whether or not synthetic oil is better than petroleum for today's modern bikes. You'll be amazed at how fast the conversation degrades into a mudslinging, name-calling extravaganza of epic proportions. It makes for an amusing way to kill the time, that's for sure.
Having said all that, what is the answer? Is synthetic oil really better than petroleum? The answer lies in how a bike engine is lubricated and whether or not a synthetic oil product is better for that process.
Lubrication Options
There are three basic options for lubricating a motor vehicle engine: full film, mixed lubrication, and boundary lubrication. Full film lubrication relies on the moving parts and the oil's viscosity to continually circulate oil throughout the system. Friction and temperatures remain fairly low.
Mixed lubrication follows a similar principle except that additives in the oil create a thin film to protect parts during initial startup. Mixed lubrication relies on actual contact between the parts to begin the lubrication process.
Finally, boundary lubrication does not involve any sort of film whatsoever. It requires contact between parts to draw oil through the system. Those parts are protected against friction by certain molecules that bond to them as the oil passes through. The binding molecules reduce harmful friction substantially.
Oil Viscosity
When you look at all three lubrication options, it becomes clear that the one consistent thing tying them all together is the role of the moving parts within the engine. Therefore, the question of lubrication comes down to one thing: viscosity. In technical terms, viscosity is a measure of a fluid's resistance to degradation when heat and friction are applied. We simplify things by thinking of viscosity as thickness. The comparison is close enough for our purposes.
When an engine is cold, engine oil viscosity is very low. That's why heavier weight oils can make it more difficult to start a vehicle in cold weather. Engine oil viscosity improves as temperatures rise and more oil circulates through the system.
Where synthetic oil is concerned, it is helpful to know where it came from. Synthetic oil is the direct result of jet engines developed for aircraft in the late 1930s and early 1940s. Petroleum based oils worked well enough until engine development started achieving temperatures that were too high. Engine manufacturers needed a petroleum substitute that would not break down under the extremely high temperatures produced by jet engines.
To that extent, synthetic oils are better than petroleum. Nevertheless, not even the most robust motorcycle engine produces the kind of heat you get out of the jet. So in a practical sense, a synthetic oil is no better for your bike than a petroleum product, unless you're running a jet turbine rather than an internal combustion engine. Once again, it all comes down to viscosity.
The best way to protect your engine is to make sure the oil you purchase matches the viscosity listed in your owner's manual. If you do that, and you maintain regular oil changes, your bike will be just fine. There's no need to spend the extra money on synthetic oils and special additive packages unless it makes you feel better.
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