I use Mobil1 synthetic in all my cars. There is no required break in period, before you switch to synthetic. Mobil1 is factory fill for lots of cars already and the number increases every year. Just don't use the new Extended life Mobil1 and never use FRAM oil filters.
http://p083.ezboard.com/ffjr1300owne...icID=536.topic (read the posts by jestal, he is a GM engineer)
The oil change light intervals vary with conditions.