10w30, 10w40 blends

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3,118
Location
San Antonio, TX
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.
 
Messages
13,132
Location
By Detroit
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).
 
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