Does it HAVE to be 'genuine' VW adblue??

[modelman]

Hi guys, My first newbie question,
I know that my 2011 bluemotion B7 estate has an adblue tank, where it it, (under the spare wheel ) capacity
( 17.5 litres ) & when to top-up, ( it tells me )
My question is this : Do I HAVE to use the genuine VW product??

I see many different makes on e-bay etc. ALL appearing to meet the SAE specs. Are they ok or do I have to pay the much higher VW price??

Obviously, VW insist that only THEIR product must be used, or engine damage will result.