I recently got the 2009 model, i'm new to bikes, but good with engines (cars and generators) in general. 10W 40 is too thin! I seriously recommend Caltex Havoline 20W 50. Everyone recommends it.
Also, people have recommended changing the fuel filter if changing the oil brand. So you might wanna do that :s sorry to be the bearer of bad news.
Extract from Wikipedia:
MULTI GRADE: (WHICH IS WHAT WE USE)
...The idea is to cause the multi-grade oil to have the viscosity of the base grade when cold and the viscosity of the second grade when hot....
The SAE designation for multi-grade oils includes two viscosity grades; for example, 10W-30 designates a common multi-grade oil. The two numbers used are individually defined by SAE J300 for single-grade oils. Therefore, an oil labeled as 10W-30 must pass the SAE J300 viscosity grade requirement for both 10W and 30, and all limitations placed on the viscosity grades (for example, a 10W-30 oil must fail the J300 requirements at 5W). Also, if an oil does not contain any VIIs, and can pass as a multi-grade, that oil can be labeled with either of the two SAE viscosity grades. For example, a very simple multi-grade oil that can be easily made with modern baseoils without any VII is a 20W-20. This oil can be labeled as 20W-20, 20W, or 20. Note, if any VIIs are used however, then that oil cannot be labeled as a single grade.
So basically, unless you're expecting temperatures below 20 degrees Celcius, dont use 10W, because our 20 to 40 Celcius heat recommendeds the 20W which is slightly thicker (50 rather than 40) and provides better engine protection. It is also MEANT for temperatures around 50 during normal running, which is what your engine runs at when its warm. So its better once the engine is warmed up (which happens within about 10 mins of running)