Ah the age old question of "what oil should I use" in truth there is not really a correct answer.
Do you understand how engine oil works? Do you know what the two numbers refer to?
Oil spec is relative to WHERE and HOW it is being used.
The important bit is the 40 in this engine, the number before refers to the start up viscosity, the engine is behaving differently when it is cold, so the oil is less viscous which means it can blast about the engine and provide early on lubrication and is makes for easier starting; once it has warmed up it will act like a SAE40. If you lived in a hot climate then you would be able to use something such as a 20w40.
At the moment a 10w40 would be fine, come summer I would be more inclined to use a the 15, BUT it then depends on what sort of driving you do.
Here in the UK, particularly at the moment with the cold weather we have been having a lower cold viscosity would be ideal. If you look at the recommended oils you will actually see they spec almost ever grade depending on your climate.