The first number is the viscosity at cold and really is relevant in the first few minutes. The top number is for the high end temps. "Thinnner" is not really the right way to describe how oil functions. Yes, you can use a summer grade however it's best to follow the owner's manual. There is no way "0" vs "10" is going to make a difference on the motor. 40 vs 30 could as the 40 weight is made for the high temps that these motors run at.
Also, unless your bike sits outside in the cold, it should always start just fine with almost any weight.