Hi
As far as I am aware, Castrol make the VW LL3 oil, but I do not think it is the same stuff. Lots of manufacturers make OEM oils, but to lower specs than their own oils.
Long life oils are not a marketing trick, but they are something for 'perfect world' conditions. So if you have a BMW 320D, used by a rep, so generally covering 100+miles on the motorway per trip at 70mph ish, 20000 miles is not a problem at all for a decent oil. Not that many cars get that kind of use though. So if you had the same car, used for a couple of miles to take the kids to school, then a couple more miles to get to work and repeat that use later on in the day, the oil isn't going to get hot enough to burn off fuel and moisture that build up in the oil, so the oil can be ruined a long way short of 20000 miles.
The fill for life autobox oil thing is wrong. For example, BMW say the oil in the ZF 6 and 8 speed boxes that they use are fill for life and don't need changing. ZF do not say that, they recommend 60000 mile service intervals. As they are the ones making the boxes, I would rather go with what they say.
Cheers
Tim