Don, I know some of the manufacturers did exactly as you said several years ago; recommended a thinner oil in order to help their CAFE specs. Down here in the warm Texas climate, I've been using 10W-40 for years and have never had a problem. And now I have two 1999 Ford products; an Escort sedan and and F150 pickup. The owners manuals for both call for 5W-30, while the new models call for 5W-20. Now I still suspect those recommendations are for the same reason; HOWEVER, I recently had the owner of an Xpress Lube place (Texaco quick oil change business that also sells all the other brands of oil) tell me that he services Texas Highway Patrol cars and that the Highway Patrol was told by Ford that their warranty would be voided unless they use the 5W-20 year round even in the Texas heat. He claims it's because of the closer tolerances and smaller oil passages of the new engines.
That sounds logical, but I don't know the fellow well enough to have any idea whether he was right or not, and I've not tried to research the topic lately, so you can take that information for what it's worth.
Anybody, besides me, old enough to remember the old annual Mobil Oil economy runs in the '50s, when Mobil bought new cars of various makes off the showroom floor each year and had a team of drivers drive them across the country and then published the mileage results? And the year when a lot of people had differential failures on Chrysler products? Turned out that since Mobil never told the manufacturers where they would buy the cars, only WHEN they were going to do it, Chrysler just put a light weight motor oil in all the differentials produced for a week or so to help their mileage results. And those differentials failed early because they were designed for 80W-90 gear oil./w3tcompact/icons/frown.gif
