I've never posted to an oil thread, but here goes. The engineers who designed and built the engine specified an oil suitable for their design. That being 10W40. The owner's manual says 10W40 API classification SF or SG.
Oils having a higher API classification meet the lower API classification. (There are some concerns with this statement: See, for example
http://en.wikipedia.org/wiki/Motor_oil or
http://www.aa1car.com/library/api_motor_oil_classifications.htm and
http://www.ehow.com/list_6120303_sae-oil-classifications.html ) Therefore, if you are using 10W40 with an API classification of SF or higher you are meeting the engineers' specs. The classification system goes ..., SE, SF, SG, ..., in ascending order. Given the above concerns, it is probably best to use SF through SJ API classification.
Multi-viscosity oils are graded as ccWhh where the cc number is the cold-start viscosity and the hh number is the hot-running viscosity. Viscosity is the flow characteristics of the oil. The higher the number the thicker the oil.
Now if you choose to use a different oil than 10W40, then you are making yourself the chief engineer with all attendant risks. That being said, one
possibly sensible alternative is to use an oil with a lower cc number than 10 while staying at the hh number of 40, like 5W40. This
should help cold-starting, and a lot of engine wear is reported to happen on cold starts. Be sure if you choose to use a 5W40 oil that it is JASO certified (safe for wet clutches). It is my understanding that all 10W40 oils of API classification SF or higher are safe for wet clutches.
A 5W40 oil may have higher consumption than a 10W40 oil while the engine is heating up to running temperature, but
probably not thereafter. I don't know, it's an educated guess,
I'm not an engineer.
The bottom line: If you are using 10W40 oil API classification SF-SJ, stop worrying and ride. If you must worry, worry about the fill-level of the oil. Keep it near the top fill line. An inspection mirror makes checking easy while you're sitting on the bike, upright, and level.
It is my belief that no engine has been damaged from the use of a modern oil of the specified type, while many engines have died from
- being under-filled
- use of a non-specified oil
- not changing the oil at the specified intervals (4000 miles/12 months)
- installing an oil filter backwards
- blocking oil passages with a home-made gasket
- faililng to re-engage the oil pump drive gear when servicing the clutch
- etc.