Buster,
Amsoil recommends their 0w-30 synthetic for these 0w-20 and 5w-20 applications and I'd agree with that, since it "brackets" the recommendations. As a number of folks have mentioned, the main reason for going to 5w-20 from 5w-30 is to save about 0.5% in corporate fuel efficiency. This also has the effect of reducing exhaust emissions by many tons per year, and Ford uses that in their "Green" advertising - you may have seen some of these ads.
If you wanted to use a Mobil 1 product in your Focus, I'd recommend their 5w-30 over their upcoming 0w-20 or 0w-30. I haven't been that impressed with the M1, 0w-30 ...their 5w-30 and 10w-30 seem to hold up much better ....
I do think you'll see Amsoil come out with a 0w-20 when there is enough market demand. I am currently running their 5w-20 in my '95 Tacoma pickup and it's doing fine. I have about 5000 miles on this batch and have burned no oil - I picked up about 0.5 mpg over their 5w-30 syn. I plan on doing an oil analysis when I get to 7500 miles ....TBN on the 5w-20 is about 10.0, which is decent.
Oil analysis of either Amsoil or Mobil 1 will show they are working perfectly and there will be no warranty issues with either one ....
TooSlick