I use Mobil 1 10W-30 in the summer, 5W-30 in the winter, the weights recommended by GM.
Mobil 1 is aviailable in 0W-30, 0W-, 5W-30, 10W-30 and 15W-30.
The number on the left shows how the oil flows when it'* cold, the number on the right, when it'* hot. I wouldn't worry too much about running a synth 5W-30 year round, in fact, that'* what I do in our other car (again, factory recommendations).
However, some of the newer recommendations are for 5W-20. To me, that'* crazy. They are using a thinner oil to get the last bit of EPA fuel mileage that they can. Seems like there'* a compromise with durability there.
Anyways, the 5W-30 should be fine in synth.