As someone who used to be a Castrol guy, I ditched them a while ago simply because of the nonsense of not making things available at a normal retail location. When the PDs came out, it was TXT which was 5w40, and it was ONLY available at a Volkswagen or Audi dealer. And I was a Castrol retailer.... even our local wholesaler could not get it (didn't even know what it was), and he actually had to go OVER Castrol USA's hierarchy straight to BP in the UK to figure out that they had some sort of "agreement" with VoA to be the sole source for 540 505.01 TXT in the USA. Seriously. How dumb is that? You could (at the time) by Castrol Syntec 5w40 in retail locations, which is what we had been buying in 55g drums in bulk for a few years already. This was 505.00 compliant (as well as 502.00.... the gasoline spec at the time, and still is for many engines).
I have no idea why the Syntec line did not get, or BP did not pursue, the 505.01 specification, but they never did get it. Since I am being trusted by my customers to take care of their vehicles in exchange for payment, they expect me to be certain that I am doing everything properly "above and beyond" and I take that task with the utmost seriousness. I drive these cars too.
So we continued to buy the Syntec 5w40 in bulk for everything else, but the PDs got liter bottles of TXT from the dealer, which was not cheap relative to the other oil. This was an issue for many new PD owners who had VE TDIs or gas VAG products previously or in addition. Because the price of a service between an ALH and a BEW was around $20 more, and I had to explain why. But, we got through it.
THEN the dealers, for reasons unknown to me, started selling Castrol SLX in place of the TXT, which was a 5w30. Now, call me old fashioned, but I had to wonder if the Xw30 oil was suitable in these hydraulic-lash adjusted flat-tappet direct-on-cam engines that were also subjected to a ~20% reduction in surface area for the valve lobes to operate on which was the whole reason for the change in specification in the first place. That was when I decided to go after a different source. And decided on a Pentosin (Fuchs) product, as they have the High Performance II that is a 5w40, meets the 502.00, 505.00, and the 505.01, as well as meets MB's stringent 229.51 spec.
The Castrol product then changed to the Edge line (displacing both the Syntec brand in the stores, as well as the SLX through the dealer) and this WAS available still through the dealer in 5w40 variety (but most dealers needed to be spoon fed the part number... otherwise they'd just sell you the Edge 5w30).
We've of course experienced piles.... truckloads... of PD cam/lifter failures since then. While certainly not THE only determinator, the use of 5w30 vs. 5w40 does these engines no favors. And I have a pretty good (meaning, not at all anecdotal) evidence of this, because I see these cars a lot. Still. Even though the last PD VAG TDI sold new here back in 2007 as a 2006 model. I see them a lot. I have a 2005 BEW J-wagon here right now for a clutch/flywheel that has well over 400k miles on it. I have a BHW in my driveway with over 200k. I have two more BEW cars behind my house awaiting untooefing with 200k+ miles.
So please understand if I won't ever recommend ANY Xw30 oil in ANY PD TDI ever, no matter its specifications or claimed "backwards compatibility". While there may be no silver bullet to make a PD's valvetrain last "indefinitely", there are certainly steps you can take to at least allow them a reasonable useful lifespan, and this is a pretty easy one. Besides, it isn't like a 505.01 compliant 5w40 motor oil is "unobtainable". I've got a drum of it sitting right here in the shop, and have been using it since 2006, and have never once had any problems obtaining it. I don't think anything will change in the near future.
Keep in mind, regarding dealers: any car as old as a 2006 model is ANCIENT in their eyes. You cannot expect them to cater to anything that would be specific or recommended for anything of that era. We buy thousands of dollars of parts from Volkswagen every month. They almost never have anything on the shelf for something of that vintage.
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New car and new to the club, and out of town, wish I had time to go through all the info here first, I would have known about the lack of customer service and or possible don't give a sht standards. 
