Oil Change Intervals
The most critical external factors to oil life are its running time and its operating temperature; on the track you need to use engine rpm as a measure of running time and engine temperature as a measure of the oil's environment.
The oil drain interval that is specified in the owner's manual is for what is called normal service. Normal service is defined as the engine being at normal operating temperature, at highway speeds, and in a dust-free environment. Stop and go, city riding, trips of less than 10 miles, or extreme heat or cold puts the oil change interval into the severe service category, which has a shorter recommended change interval.
When the manufacturer gives a mileage/time criteria for oil changes they're telling you that if you drive every day, but (say) for only short trips to work, your driving habits put you into the severe service category and you should use time, not mileage, as a guide. So you may go 100 miles a week but do it in ten trips.
Motorcycle riding is often different. You usually ride for pleasure, but maybe only once a week outing for 100 miles, one trip (normal service.) So you should use mileage, not time as your change criteria.
Further, manufacturers are saying to change your oil even more often than the mileage interval stated in the manual because your motorcycle rarely experiences only normal service conditions.
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, because I ride infrequently for 200 mile jaunts - my choice - is to change my Mobil 1 15W-50 automobile-specific synthetic oil at 3,000 mile intervals.
Oil Temperatures
If the manufacturer supplies the engine with an oil cooler, it needs an oil cooler. Otherwise not, unless you have made significant power-related modifications. Air-cooled engines are designed to run hotter than water-cooled engines, but the rules for oil temperatures are the same for both engines. YOU may be uncomfortable at the higher engine temperatures but the engine has been designed with adequate materials and thermal expansion clearances to run just fine.
From the engine point-of-view, you should expect to see indicated oil temperatures between the gauge midpoint and three quarter mark with occasional excursions to full scale. That’s how the manufacturer selects the gauge mid-point. If the oil temperature is too cool the oil viscosity is higher so you will loose power due to pumping losses.
From the oil point-of-view, you need to maintain lubrication and a high enough viscosity when hot to produce an adequate oil pressure. If the oil temperature is too hot, the oil oxidizes and breaks-down more quickly.
That’s why you should run a synthetic oil, to give you a safety margin against oil breakdown. A regular oil will begin to lose its film strength at temperatures above 220°F, while most full-synthetic oils can be safely used at temperatures as high as 300°F before lubrication-related damage becomes a concern.
As a rule-of-thumb your oil temperature should be kept below 240ºF if you want to change petroleum-based oil at the recommended intervals.
For every ten degrees above 240°F, cut your oil change interval in half. A good rule-of-thumb for track use.
Race car builders usually design for oil temperatures between 230°F and 260°F in order to get the best power. Above that range, engine reliability and oil life becomes a factor in racing.
