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A synthetic 0W-20 is ideal for your situation. Contrary to popular beliefs, a 0W-xx oil is not "thinner" than a 5W-xx oil. The first number refers only to the oil's characteristics at very low temperatures.
If I lived in Minn. I would certainly run a 0W-xx oil in the winter. Mobil-1 0W-20 is an excellent choice for you. The only reason to switch to 5W-20 come summer time would be if it is easier to find or significantly cheaper.
For a warm climate like South Texas there is little advantage to the 0W-xx rating. It isn't going to hurt anything, but probably will not do much for your either.