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The main reason for using a higher viscosity 50 weight oil is that the oil looses viscosity because it also lubricates the transmission.

The central dogma of motorcycle oil manufacturers and distributors has always been that motorcycles put different demands on their lubricants than do automobiles. In particular, they point to the facts that motorcycles run at higher temperatures and use the same oil in their transmissions as in their engines. The transmission gears supposedly put extreme pressures on the oil molecules, thus causing the long oil polymers to break down. High temperatures can have the same basic effect, as well as additional effects such as the increase in oxidation products.

When the size of the oil polymers decreases ("cut up by the transmission gears," as at least one manufacturer claims), the oil thins. In other words, its viscosity decreases, as well as its ability to lubricate properly. For example, what started out as a 40-weight oil could effectively become a 30-weight oil, or even a 20-weight, after prolonged use.

The viscosity of synthetic-based oils generally drops more slowly than that of petroleum-based oils in the same engine.

Here's the result of one test.

Castrol GTX, a non-synthetic car oil at 800 miles showed a relative viscosity of 0.722, meaning it had retained 72% of its original viscosity. Or, if you want to look at it the other way, the Castrol had lost 28% of its viscosity after 800 miles of use in the motorcycle.

Just for comparison sake, they also tested the viscosity drop of the same Castrol GTX oil after use in a 1987 Honda Accord automobile. At 3,600 miles of use, the Castrol GTX showed a relative viscosity of 92%.

So a motorcycle is definitely a more severe operating environment than a car so the oil change mileage interval should be shorter than for a car.

In the same test, since Mobil 1 car synthetic oil had retained so much of its viscosity after the 1,500 mile test, it was the only oil allowed to run longer in the motorcycle. After 2,500 miles, the Mobil 1 recorded a relative viscosity of 79%.

One more point. If you buy a motorcycle-specific synthetic oil it's no guarantee that you can extend your change interval. There were two motorcycle oils tested, Spectro 4 (petroleum based) and Honda HP4 (petroleum/synthetic blend). Both the Honda HP4 and Spectro 4 had lost over 30% of their viscosity at 800 miles, and over 35% at 1,500 miles.

So, my choice is to use Mobil 1 15W-50 automobile-specific synthetic oil and change it at 3,000 mile intervals.

Your oil viscosity selection chart in your owners manual tells you to change to a thinner oil if the expected outside temperatures are lower. As you can see from the chart, a single modern motor oil with viscosity enhancers can be used over a wide range of ambient temperatures and still maintain adequate oil pressure. So oil viscosity not a sensitive parameter.