Innumerable people have done exhaustive reports on this subject, a few of them excellent. But, for all that, there haven't been many from the point of view of the career Japanese motorcycle mechanic.

Viscosity. It is generally accepted that viscosity is an oil's major friction-fighting property, though this has become less important. Plus, consider that viscosity thickens when the oil is cold and thins when it is hot, exactly and perversely the opposite of what the engine needs. This is why multiviscosity oil was developed. Multivis motor oil addresses the changing viscosity problem by being chemically treated to thicken less when cold and thin less when hot. Pretty neat, this. Multiviscosity oil isn't oil with multiple viscosity behaviors; rather it's oil whose viscosity is less affected with temperature. It has a high "viscosity index", i.e. stability. On a graph, the multi-vis is depicted by a line closer to horizontal, while all single-weight oils have more vertical lines of change.

Volatility. Another one of motor oil's inherent weaknesses is that it is quite volatile; some of it boils away into a gas when churned and heated. The industry standard allows up to 15 percent by volume, and powersports engines are mostly right at this limit. Few people realize oil vaporizes, but it is in fact why oil level must be monitored more closely on motorcycles. Synthetic engine oil is far less volatile, by the way. One of its many advantages.

Antibodies. Conventional petroleum motor oil has a lot of added chemicals in it to make it work better than its base stock alone could manage. These include anti-foam agents, anti-oxidants, and of course detergents and dispersants. However, these enhancements wear out before the oil itself does, way before. Synthetic motor oil by contrast has (and needs) few things added to it in its formulation, enabling it to do its job better, longer, cooler.

Synthetic oil and oil consumption. Many riders have experienced initially high oil consumption when switching to synthetic motor oil. Oil engineers admit this happens. They confirm that when a synth oil is introduced to an engine that has been running mineral oil, sludge and varnish that has built up around engine parts is washed away, instantly changing engine clearances. This is only temporary however. By the second or third oil change things will return to normal. Though a well-known phenomenon, this has scared many away from synthetic motor oil.

The Samurai's war with the EPA. Despite synthetic's advantages, petroleum vs. synthetic isn't your most important oil decision. Not even close. The most important choice you will make is to use powersports industry endorsed motor oil in your Japanese motorcycle. Endorsed oil? Yes. In an attempt to reduce exhaust emissions, the Environmental Protection Agency (EPA) in the 1990s forced the engine lubricant industry to begin adding friction reducers designed to improve the fuel economy in car engines. However, friction-modified oils are incompatible with the oil-bathed clutches and sprags that are used in Japanese motorcycle engines. To counter this, the Nippon leaders of the powersports industry in 1998 established a special powersports motor oil standard that overrides U.S. "energy-conservation" requirements. This new standard, JASO MA, is a watershed exemption from the EPA oil and it identifies the only oil you should be using in your Japanese powersports vehicle. Look for the MA designation on the bottle. This is one of your most critical choices regarding engine oil.

Shifting. Smooth shifting in a Japanese transmission depends heavily on optimum clutch function, so adjustment using the factory's method is important, as is also careful clutch cable routing. But many are unaware that clutch function in these engines is also largely dictated by the engine oil. Oil level is especially important in this regard. A slightly high oil level negatively affects shifting. And as you would expect, improved shifting is the single greatest real-world benefit of using a synthetic motor oil, mostly due to cooler clutch operation.

The importance of maintenance. All of these things, which I have dealt with in previous articles, are very important: the superiority of multivis, being aware of volitility, the relatively short life of modern oils, synth oil's many advantages, the MA stamp, and engine oil's direct effect on shifting. But, if you run your bike out of oil -- well, forget all of that. None of these things will help you. My experience is there are always some folks who will let their oil level get too low. It's way too common. Check the level often. Run the engine for a minute before determining the final level during refilling, don't just measure it in. If you have an oil cooler, best practice is to check the level before the cooler has had time to drain down into the crankcase and give a falsely high reading. Because, the simple fact is, the single most important thing to remember about your engine oil is keep enough in there. Ask any mechanic. In the repair shop, it is many times more common to see issues connected with the lack of oil than it is all other oil variables combined.

Give that some thought.

Also see:
More on motor oil
Powersports motor oil