Yes that's the stuff (an oil with VW 504/507 approval - mobil,motul,shell,fuchs etc all do their own equivalents). Lots of post about oil...
Certainly a ploy by Castrol to get you to buy 2 4ltr cans. Yes the car takes 4.6 litres. You can always buy in bulk, but I suspect it's 25ltr minimum.
I got an oil & filter change by National Tyres. Like you I'm on long life but wanted an interim oil change. I was charged £28.80 for the oil change (book online) and yes that is with the proper long life oil - Castrol Edge 5-30 - and includes the filter (obviously not a VW filter - but nothing stopping you supplying the filter if you're that worried - it would still be cheaper). I was going to do it myself but considering you can't even buy the oil for £30 I got them to do it (I did watch them like a hawk though

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The car's still full of oil and it hasn't blown up!