The only logical reason I can see is that Ford must have their hands in it. Most manufacturers do this hoping to trick consumers into getting service done only at the dealerships. Rest assured that you can use any lubricant that meets the specs for that particular application without warranty issues. This was an issue a while back but the government got involved and put into affect the Magnuson-Moss Warranty Act in 1982. Read about it here.
http://www.consumer.ftc.gov/articles...ne-maintenance.
If you have a Powerstroke and would like to know what AMSOIL products to use let me know what you have and i'll get you correct information.