I found this on the CB500X forum.......the guy seems to know what he's talking about!
10W30 is a multigrade oil. When it is cold it has a viscosity of 10 (the weight of the oil) which means that it is thin. Exactly what you need in a cold engine. When the temperature is +100°C the oil has a viscosity of 30 (somewhat thicker). Again: exactly what you need with a warm engine.
Switching to 10W40 is not a disaster: your cold engine runs with the same thin oil (viscosity 10) but when it has +100°C, it will run with a thicker oil (40). A thicker oil could be used when the engine is somewhat worn out as it offers a bit more protection. Remember: thicker oil is harder to pump around.