The Mobil 1 0w-40 meets the A3 European standards but is not as shear stable as the M1 10w-30 or the 15w-50 flavors.
The most widely available 5w-40 is the Group III Shell Rotella Synthetic which is a Heavy Duty Diesel oil or for work trucks. Mobil Delvac 1 5w-40 (Mobil's best oil)is a Group IV but is not as easy to find as the Shell Rotella T.
From what I have seen about the 5w-20 (which is recommended by Ford and Honda), the base stocks and the additive packages are better than the heavier weights such as 5w-30, 10w-30. The Pennzoil has a good dose of moly and is primarily Group III basestock. The Motorcraft brand (Conoco/Phillips made)looks to be semi synthetic. Check the UOA section, these 5w-20s have shown good results.
If you are worried about warranty issues, use either of the two above referenced oils.
[ December 17, 2003, 04:12 PM: Message edited by: John in the ATL ]