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The 5W is used for energy conservation during cold engine operation. A lower weight also allows for oil to flow up to the top of the engine more efficiently in cold weather. The 30W is the maximum GM recommends for any gas engine they now manufacture (the Honda engine in the VUE required 5w20).
These requirements are per SAE, and it means in cold temps, the oil will flow like a 5W, as the engine warms up it will flow like a 30W. Some type of index viscosity additive is used to make this all happen.

Using 40W in ANY modern GM engine will most likely cut its life span. In addition, I don't believe any 40W or greater oil passes GM's required spec for the H3 engine. If you ever have a major internal engine problem, and a GM field rep gets curious, he/she may send a sample out for testing. In two days, they can tell what oil is in the engine.

What oil you should use will be debated until the end of time, just like when to change your oil and which is better Chevy or Ford trucks. So, I'm just telling you what GM requires, that is in writing in the owner's manual, and to understand, using this recommended oil WILL not cause engine problems. I always have used 5W-30 in my Vettes and lived in the south, and had no problems.