Edit: I stand corrected - it's 5W-20 synthetic blend (based on a few recepits from previous oil changes that I just reviewed). I had always remembered using 10W-40 in the 70's, or 10W-30... so I was surprised when they said they used 5W-20 (I remembered incorrectly and was mistaken on my previous post).
Ford's viewpoint
5W-20 oil is a thinner oil with lighter viscosity that creates less drag on the crankshaft, pistons and valvetrain. Additionally, the oil pump can pump thinner oil more easily, improving oil circulation. Machined internal engine parts are more precise than the parts of 20 years ago. Continued in next link below...
Oil additives are not recommended as noted in the owners manual.
https://www.fleet.ford.com/FFH/Why5W20Oil05_23_06.asp?News=Rental
Why 5W20 Oil?
Some customers are reluctant to follow Ford's recommendation to use 5W-20 oil in their engines based on the incorrect assumption that Ford and other Auto Manufacturers only recommend 5W-20 oil in order to increase fuel economy. Using 5W-20 oil can increase fuel economy by about 6/10ths of a percent compared to 5W-30 and more if you are currently using a higher viscosity oil. This equates to an additional savings of 125 million gallons per year when used in all applicable Ford vehicles. Since its introduction in the 2001 MY, 5W-20 oils have saved up to 640 million gallons of gasoline in the U.S. or an equivalent 5.6 million metric tons of carbon dioxide emissions.
https://www.fleet.ford.com/FFH/Rental05_23_06.asp
Different opinion:
http://synthetic-motor-oil-air-filters.com/amsoil/5w-20-5w-30-0w-30-recommendation.htm
http://synthetic-motor-oil-air-filters.com/synthetic-oil/0W-20-synthetic-motor-oil.htm
http://www.ultimatesyntheticoil.com/articles/ford5w20.htm
Who knows what the truth is, other than everyone needs to sell their product.