Use what the owner's manual calls for, based on outside temps. I know that the owner's manuals for both of my Benzes suggest 10W-40 as a year-round oil. However, I use 15W-40 since it is a diesel rated oil.
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