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.
Now can I buy any 0w40w synthetic oil the ams oil makes or mobile one makes or does it have to be for a 4 stroke oil and or wet clutch compatabile? Thanks ahead of time for the help I'm pretty new to atv's.
There is no wet clutch so you can't use that nor 2 stroke oil. Your manual states what the requirements are but it pretty well covers all oils now a days. I'm not much in believe that oil should have additives though.