This is reprinted from a Ford Fleet newsletter linked on Bob is the Oil Guy’s website. The link no longer works as it is 12 years old.
"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.
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. Any increase in fuel economy may not be noticed by the average motorist. Machined internal engine parts are more precise than the parts of 20 years ago. This means that clearances between moving parts are smaller and more exact. Thinner oil such as 5W-20 can flow more freely through the engine while still filling the spaces. Thicker oil is harder to push through the spaces between the parts. This causes the oil pump to work harder, which in turn increases oil pressure while simultaneously decreasing oil volume. A lack of oil volume results in a decrease of lubrication and cooling, which may decrease engine part life.
The lighter viscosity of 5W-20 oil flows faster at start-up compared to higher viscosity oils, which helps reduce engine wear in critical areas by lubricating parts faster. Valvetrain components at the top of the engine require immediate lubrication at start-up.
Oil additives are not recommended as noted in the owners manual. The American Petroleum Institute (API) certifies that oils such as Motorcraft 5W-20 already contain the necessary additives for friction, detergent, etc… The addition of additives may interfere and react with the additives already present in the certified oil."