Watching downloaded movies?

I downloaded a couple of movies to my itunes and have used it to watch them just fine, but when I try to sync them to my ipod 160G classic it says I can't watch them on this ipod. Is it because I downloaded them and didn't buy them from apple? Cause the movies and TV shows I have brought on itunes sync to my ipod no problem. They were downloaded as MP4's

Thanks for the help.