Use the 5W20, it's common knowledge that it's more than up to the task when it comes to engine protection, especially since most of engine wear occurs at engine start up, and thinner oil gets circulating faster than thicker oil. There's no reason to use 10w30.
Also using synthetic oil doesn't mean you get to extend OCI (oil change intervals). It just means that you're using a better than required product.
yes/no...synthetic is tougher better oil...but
1 cost more...if you r changing you oil evey 3K you are wasting the 7K protection syn oil gives so u may as well use GTX or other fine regular oil
2 5w 20 is too thin and the 20 wt not enough protection...ford factory uses it to squeeze oit every 1/10th of a mpg for fleet average [and with a new engine]
i like 10 w 40 castrol gtx synthetic and change at 5K
I know my brother always uses Royal Purple 5W20 Motor Oil in his 02 (http://www.brenspeed.com/1520.html) mustang from BrenSpeed.com because I always receive the shipment in the mail. He has been a mustang enthusiast since he was a little kid so I trust the products he uses in his car. He said he doesn't use synthetic because it costs more money.
you should be fine with changing over to synthetic although keep in mind if your just driving it with no problems then why change? also if you switch to a synthic oil and you've been using conventional there might be some issues with leaking.