5w30 vs 10w30

My owners manual says that I can use either weight as long as it's 0 degress or above. So, knowing that what would you run? The dealers say that I should run 5w30 all year. Some other people say I should run 10w30 all year. And yet other people say I should run 5w30 in the winter and 10w30 in the summer. What I don't understand is how can 5w30 and 10w30 differ when they get hot since they both are 30 weight on the high end? I have this question when people say that 10w will give me a little better protection in the summer. Is there really a difference? Yes, I live in Wisconsin, so the temperature does drop in the winter. Oh, my oil cap says 5w30 on it so does this mean that 5w is better?

1998 Z71
5.7L

According to any mechanic I've talked to, the advantage of 5W30 oil is slight gas mileage improvement over 10W30 in the summer. Also, using 5W30 in the winter allows the oil to circulate more quickly on startup, which as we all know is when the most damage to an engine can occur. I hope this helps you out. Personally, I have used Valvoline DuraBlend (mixture of synthetic and organic oils) in my 2000 Silverado 5.3L since the second oil change; 5W30 in the cold months and 10W30 in the summer. My truck has close to 50K on it and it runs as good as the day I picked it up at the dealership... don't know if it's 'cause of the oil, but I'm sure it didn't hurt!