Well, obviously, the big thing is to keep good records. Besides that, I think that the dealer is more prone to honoring claims if you come into the dealership with your records ready, ask questions that display your technical knowledge on the subject, and if they start to balk, bring up the Magnason-Moss act in conversation, perhaps how you were discussing it with a lawyer friend.
All of this is much simpler if you did change at a recomended OCI, in part because the average service manager knows so little about oil that he won't realize that appropriate oil analyses on longer OCI's will hold up in court.
Also, I brought up AMSOIL's own waranty earlier, and I have just become aware that they are know to refuse coverage if they believe that the engine in question has an engineering defect.
As to the 5W20 for CAFE requirements, I'm undecided. Motorcraft 5W20 is a pretty stout oil, actually a blend, and it turns in good wear numbers (some engines to 100k+). Also, I can't imagine why Ford would TSB the 5W20 retroactively if it wasn't at least partially for improvements in other areas besides mileage, the EPA doesn't count what happens to mileage on a 1995 Taurus when figuring out CAFE points, does it?