As Longston said, use what your manual calls for. My VW owner's manual says to use 5W-40, but 5W-30 can be used if 5W-40 is not available. So "40" is the perferred weight. My '80 SD's manual says to use 15W-40 for temps above 23 F. Below that, use 10W-40 or 50. Again at least "40". Domestic and Japanese cars call for anything-30. It seems everything from Europe calls for the heavy oil. Look what Porsche is factory filling their cars with, 0W-40 Mobil 1. There's that number again, 40.
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