Ford recommends using 5w-30. On newer vehicles 10w-30 would only be needed in a desert. And if you run a true synthetic (mobil1, amsoil,etc. not castrol) 5w-30 you can run it all year in just about any climate. 10w-30 can mess up the VCT system for ya'll that are unfortunate enough to have it, for us pre98ers, I'm not sure what it can mess up, but when I started using mobil1 the engine ran a bit better.
