VW 504/507 may be a proprietary oil standard, but it is open. That way, any lubricant manufacturer can submit their product to VW for evaluation and, upon passing their tests, obtain approval. You end up with a system where the consumer is free to use any brand of oil.
considering you can buy them in handy 1 litre bottles, and particularly if you live in Sydney.
Castrol is Volkwagens's technical and marketing partner, but from a business point-of-view, I'd think they have no reason to formulate their 504/507 lubricants any better than the criteria already laid out in the oil standard.