I'm currently running amsoil synthetic. My first change to synthetic was at about 3500 miles. I was told (and read in owners manual) that 10w30 is acceptable if not below freezing. I did notice a difference when I changed from 5w30 valvoline to 10w30 amsoil. It was a bit more sluggish, not as good on mileage. I gave synthetic a second chance, and went with the amsoil 0w30.
The stuffs awesome. I still replace my oil every three thousand, but they say you can go up to 12000, you just have to change the oil filter,
I didn't like the looks of my oil at 3000, so i changed it anyways, and continue to do so. 5w30 is acceptable too. You can read up on 0w30 by amsoil at
Amsoil Website
I haven't tried another synthetic. originally I tried it because it's all my shop uses, I then found out you can get an account at amsoil and get a great discount. Thats when I switched to 0w30