I wanted to stick with the 5W30 weight as that is what GM strongly recommends. They say never to run a thicker oil other than 5W30.
It's important to recognize that there are considerations OTHER than engine longevity that can drive a strong OEM recommendation.
For example, almost all the 20-wt oils are recommended because the slight fuel economy benefit associated with them was used in achieving CAFE measured performance. EPA then wants to know how manufacturer will prevent customer from using thicker (lower mpg) oil. OEM goes to EPA with all the scary manual stuff end firm recommendations against the thicker stuff, EVEN if it might make the engine last longer.
Qualifications: I work as an engine engineer for a large engine company, and have had conversations with world-class tribologists about this kind of thing.