5W20 versus 5W30

Here in the USA, the recommended oil is 5W20.  But in other areas of the world (everywhere outside of North America), for the exact same engine, the recommended oil is 5W30.
 
Can someone explain to me how this makes any sense? 
 
I must say, I'm leary about 5w20 because of how thin it is.  Yes, new engines are tighter.  Yes, I'm sure their making better motor oil now than ever.  But what about protection once your up to operating temperature, or running through the mountains, etc.  I don't want my new Mazda to be part of the guinea pig experiment to see if engines can really last a long time running with 5w20.

Here's a good response to this question:
 
http://bestsyntheticoil.com/dealers/amsoil/5w20-cafe.shtml
 
 

i know exactly why.

5W-20 oil has a lower viscocity (its more watery) than the 5W-30.  In order for Mazda ( and all the other companies who recommend 20 over 30 ) to meet epa requirements for their SUV milespergallon, they had to choose the 20. because it saves gas, around 0.1% if i remember.   Its a negligible amount, but added up to all the cars that Mazda produces. it makes a difference.

In short. just use 5W-30.  I do it.  id say 80% of enthusiasts do it.  it causes less wear on the engine (due to the thicker oil). 

hope this helps

automan, that is a great link, thanks for that.
 
sin6racer, I insist my oil change people put 5w30 in when they change it (they often say,"but the cap says 5w20", and shake their head as if I am asking for something weird).  Especially because I don't live in a Minnesota-like winter climate location, I agree 5w20 is too thin.