I use Mobil 1 full synthetic 5w-30 in our Cobalt and Silverado, it really isn't very expensive when you can get 5 quart jugs for under $30 if you watch for it. We bought these cars new and intend to keep them for an extended time so I feel its worth it. I use the oil monitors in them and change the oil at 30-40%, this is pretty close to 5k and 4 months of our driving. When we bought the Camaro I ran Mobil 1 in it too and when the original engine blew up (nitrous issue) the inside looked new so I was really sold on it then. After the break in of the new engine I've been using Royal Purple 10w-40 since the bearing clearances are slightly larger.
For my older vehicles I've always used a factory recommended viscosity and used a high mileage oil because of it being semi-synthetic. I'm sold on it. It only costs a few extra bucks and I get piece of mind. I've done thousands of oil changes over the years of working as a tech and my oil almost always looks better coming out then the cars I work on. I know the majority of cars I see at work use dino oil and don't get their oil changed as religiously as mine, but most come out with the consistancy of water. My cars always just looks like dirty oil, I just don't think it breaks down as fast.
Not saying this is any scientific proof, but its enough for me to spend the extra few bucks.