One mechanics opinion about oil

I just had my oil changed at the Ford dealer (first 3 oil changes free because I bought it there) and when I picked it up, the service department manager told me that I was good for 3,000 miles or 3 months. Talk about selling service where it is not needed. The F150 owners manual says to change the oil according to the on dash display, or one year, or 10,000-whichever comes first.
I think that I will go by the Ford engineers recommendations. I do shorten those numbers up a bit, but not by much.