I'd imagine wear would be less with the 40 weight, at least in my cars. If you are thinking about the flow being better in 30 weight oils as being more crucial than a viscosity that allows more pressure on the film, I disagree. Of course engine design, OCI and driving style will affect wear ultimayely, but 40 weight is the way to go, imo. Check out wear on 15w-40 oils too.
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 ]
Thats a really, really, really broad question which has no answer. First bc one 5W-40 oil may be different from another brand-same with other grads. Also the type of vehicle,driving conditions and climates will make a difference. You propably will not get a lot of helpful information-the question is just too general. Just stay tuned and you may find the Holy Grail