Dexos or NOT!
 

I notice the Nissan owners manual specifies an API rated oil for the engine. GM recommends Dexos. Must be a marketing move.

Of course in a pinch any API rated oil will be just fine says GM.

I will probably run SAE 30 in the summer, 5w-30 winter and 10w-30 spring and fall. As I still have a ton of the Auto Zone .99 oil left over,

Indeed a marketing move, it's probably because its what they sell at the dealership and want you to buy it. Follow the money.