If that Castrol variety is made in Europe then go for that one all day long. Group IV oils need less viscosity improvers and are more stable than hydrocracked group iiiís like Mobil 1. I know the 0w-30 and the 0w-40 are all group IVís.
Castrol 5W40 TD is not Group IV.
Currently castrol offers only two oils in Europe that are true synthetics, or Group IV: Castrol 0W30 LL04 and Castrol 10W60.