The Magnatec 10w-40 semi synthetic is much like any other well branded 10w-40 semi out there, if a little over priced compared to its competitors.
To the OP on a Impreza Sport it will be just fine with regular changes, certainly better then the Halfords own 10w-40.
If in the future you want to go better then try a 5w-40 synthetic.
Cheers
Guy.