I agree with Bo, about the "squeezing" 1 mpg out of the engine. The big push for 5W-30 is for fuel economy only. To the normal person, what's 1 mpg better fuel economy. However you multiply the 1 mpg times the number of cars GM builds, it adds up. Many owner's manuals even list 5W-30 as only good for up to 60 f. only. My father-in-laws '93 Subaru says not to use 5W-30 for temps above 20 degs. Personally, I'll take superior long term protection over fuel economy.
1994 MB E320 Coupe (199,500 mi)
1992 MB 190E 2.6 (173,000 mi)
1983 Porsche 911SC (148,000 mi)
2004 Volvo V70 2.5T (198,000 mi)
President of CT/Westchester Section