Dealers will use different oil for T&D - its just us forum folk prefer using longlife 3 as it gives the best protection to the engine whatever your service setup
Hmm theres little way to prove that what they said is true, but even so 5w40 is VWs recommended oil for T&D and not 10w40 on these engines.
(what I copied was from the TPS oil finder who are a division of VAG).
Castrol GTX is 'lower' quality than magnatec - its more of a general oil as opposed of the magnatec which as a more specialist oil, offers better cold start protection etc. GTX also costs less than magnatec.