It doesn't have to be Castrol anything, y'know. There are other brands on the market who make oil specifically for the PD tdi engine. Castrol are just a brand name of a company who make oil (and not a particularly good one at that in my opinion).
What you need to look for is the latest VW?Audi spec. That is VW504/507 and a weight of 5W 30. Anything else is really irrelevant as it's manufacturers blurb and advertising. A personal favorite oil is Gulf GVX and you can buy it from other suppliers aswell. Mobil ESP is also very good, as is Fuchs Titan GT1.
After that, you can use anything you want really, but just make sure you buy with the spec outlined above.