10w30, 10w40 blends

[jorton]

I noticed some 10w40 blends have the European certification but 10w30 of the same brand does not have it.
So what's the difference between the two grades? One brand I was looking at is Castrol.

 

[Motorbike]

I believe you are speaking of ACEA A3 that requires an HT/HS of 3.5 . Most 30wts cannot achieve this rating because of the Vi .

 

[TallPaul]

I was told by an oil company rep that 10w40 would have a thicker base oil than 10w30 of the same brand. This seems confirmed in that Exxon Superflow 10w30 and 10w40 have HT/HS of 3.1 and 3.7 respectively, although the starburst/energy conserving rating may have messed with the HT/HS of the 10w30 (the 10w40 dosen't have these ratings).