I'll stick with the Motorcraft synthetic blend since that's what the car came with and Ford can't weasel out of the drivetrain warranty if I keep using it!
It's more important to stick with a type of oil then anything else. The type of oil you use will determine the wear patterns your engine gets. Switching back and forth will cause the patterns to change which can eventually wear things out prematurely. For all intensive purposes, a full synthetic is the best bet. However, a synthetic blend is a nice compromise too. You might not get every little last benefit of a full synthetic oil, but you get quite a lot of them such as reduced wear, better flow, and longer life.
Since I got a maintenance plan with my car, I just have them change the oil every 3K because that is what I paid for. I feel I am not adequately getting my moneys worth if I wait till the car tells me to change oil. Funny part is, all 3 dealers that have touched the car always put the sticker on the windshield at 3K intervals as well.