Interesting, most forums I've read DON'T recommend using dressing on serpentine belts.
Any chance of the alternator, water pump, or power steering pump causing the squeak?
It wasn't my personal recommendation to use belt dressing, it was just an idea to try "if all else fails". Trying to come up with a cheaper solution since he spent money at Ford with no success. I also said it's "suppose" to eliminate belt squeals, not a garuantee from me that it would. On the alt, water pump or power steering pump causing the squeak I ruled those out from his OP, since he stated he had the vehicle looked at by a Ford dealership, and we all know how they(Ford) would have loved to take his money by replacing one of those components as the problem, and even they couldn't find the source of the squeal/squeak. Ford doesn't like to turn easy money away. He also said when he changed to a new belt, the noise went away for a short time till that belt wore a little. My original thought was, buy and try the better belt(Goodyear Gatorback). Interesting stuff on the links you supplied and I knew about the gummy nature of belt dressing that is why I told him to follow the directions on the spray can on how and where to apply it. You wouldn't want to over spray that stuff on any other parts. Thanks for the links!