Originally Posted By: Scum_Frog
I have moved on to using 0W40 as my regular fill as it is the newer oil and is API SN. Does it really make a difference, probably not
Just to clear up any misconceptions regarding the API ratings between the 0W30 and 0W40 (as was discussed many times in various places on this board)... There is NO significant formulation difference between the 0W30 SL stuff and the 0W40 SN stuff. This is because API restricts certain additive levels for xW20 and xW30 oils in their SN spec, but leaves the xw40 and xw50 grades "exempt" from such restrictions.
Therefore, in the case of the two Castrol oils (0W30 and 0W40), they have very similar additive levels, and neither one is "newer" or "better" than the other in terms of formulation. They both meet most of the same relevant manufacturer specifications.
However, due to API restrictions, they cannot label the 0W30 as SN, so it must remain as SL for the rest of its useful product life cycle, whereas the 0W40 can be labelled as SN because it is exempt from the above restrictions. This is all despite the fact that these oils have very similar additive levels / packages.
Therefore, ignore the SL and SN on the Euro oils. There is no difference (other than viscosity / HTHS) between the two oils.