It's ironic and unfortunate that the auto companies try to use the warranty meant to protect the customer against them. Frankly, though, if you use a 5W30 oil, I think there's about a 1% chance you will have a problem. If you can't see your way to using SynLube, you should use Amsoil 2000 until you leave the warranty period. The difference between 5W20 and 5W30 is a very large one.
I had a 2001 Ford Focus that I converted to SynLube @ 1200 miles, (engine and transmission) and Ford never tried to deny any of several repairs the car required (electronic components)because of the oil I used.
The new Duratec engine is one of the first engines designed with thinner oils in mind though I'm not sure just what changes such a thinner oil would dictate. SynLube has developed a narrower purpose 0W40 oil for these engines. I say narrower because such an oil cannot be used in diesel engines.
In any case, Ford would have to prove that the oil caused the problem if they are going to deny warrenty service. My guess is that they would probably use the fact that you are towing equipment with the Focus as an excuse before they looked at the oil.
I guess the choice we all have is whether to use an oil that will actually eliminate the need for oil-related repairs versus the reassuring feeling that if the engine needs warranty repairs they will be performed without question.
I come down on the side of using the proper oil because if a major repair becomes necessary because the 5W20 oil was insufficient to the engine's needs, the engine will never be the same. The sophisticated construction techniques employed in the Duratec 20 engine cannot be duplicated at a dealership and the engine depends upon the method of construction for its reliability.