From a couple data points on my FSI the moly-containing 10w40 didn't show any dramatic improvements or negatives compared to the 5w40.
As I understand it, moly and the esters are in competition for surface area, so large amounts of moly are needed to attempt to achieve the friction modifier benefit. As AJ asked, does the moly actually benefit in this ester-containing formulation? another question I have is does it benefit without significant build up of acids due to having too many sulphur containing additives in the oil?
In comparison to 5w40 UOAs, the following positives and negatives were observed (but some of these differences are small and probably well within the "noise" of the testing or could be accounted for by change in weather or driving or other variables):
Positives:
Possibly slightly smaller iron levels
Possibly slightly better shear stability
Negatives:
Possibly faster TBN reduction and TAN increase(i.e. the 10w40 might be a shorter drain oil than 5w40)
Some 10w40 vs. 5w40 discussion here and my UOAs are posted in this thread as well:
http://www.bobistheoilguy.com/forums/ubb...029#Post2005029
At this time, I would choose the 5w40 over the 10w40 due to the benefits of possible less acid accumulation, the wider temp range (possibly giving a slight mpg bump in lower temps during warm up period) -and possibly reaching the HPFP cam follower and/or turbo slightly faster in cold temps???- But more data would help to better define the benefits and drawbacks. They are so close that it won't make that much difference which you choose but personally I think the 5w40 is a better fit for this engine.
