You can switch back and forth from synthetic to non synthetic with no ill effects at any time.
Your engine was designed with 5W30 in mind. Don't run anything heavier, even in summer.
There is no oil available that is too thin at start up. You can move to 0W30 or even 0W20 with no problems. Even 5W20 will be OK.
Literally any oil on the market today that meets GF-4 and SM specifications and of the right viscosity will work wonderfully and certainly go at least 5000 miles between oil changes. Today's conventional oil (dino, or non synthetic) are very nearly synthetic anyway. They have to be to meet the specs. The performance between dino and synthetic has been narrowed greatly. The biggest advantage to synthetic at this point is in temperature extreemes and if you are trying to go for extended drain intervals of 10,000 miles or more.
If you are really interested in learning all you can about oil, hang out here:
www.bobistheoilguy.com 
