I'd say that if that car still has G13 in it, only ever add VW Group G13 - why go to the bother of mixing coolants when the original stuff is available at roughly the same price.
It is well known that only 1 or 2 companies actually manufacture the coolant concentrate - so why risk buying it maybe from someone that gets a 3rd place to sort of make almost the same stuff, well ends up the same colour?