Kelly99 -
If you look at the product data sheets, you will see a VERY low pour point on 0w40 Castrol. That indicates that it has a fair amount of Group IV (most likely).
You are also correct that Castrol 5w-40 is a weak product compared to the 0w-40 product.
There are some primarily Group III+ oils that are exceptional today, like Mobil 1 0w40.
Therefore, group isn't always important. Nevertheless, Castrol 0w40 is a better product in European cars (meets more Euro specs) than Castrol 5w40, and based on pour point, it probably is more of a traditional synthetic, for whatever that is worth.