I tried to edit my previous post but time ran out. Here's my adendum to my previous post:
Directly from the chemistry department at Wolfsburg University (ok, I'm embellishing):
G-11 (the old blue stuff) can be mixed with G-12+
G-12 can be mixed with G-12+
G-12+ can be mixed G11, G12, and G12+
G-12++ can be mixed with all "G" coolants (G11, G12, G12+, G12++, G13)
G-13 can be mixed with all "G" coolants
NEVER MIX G-11 WITH G-12!
For the record, Glysantin (BASF) is the OEM developer and producer for VW/Audi coolants, hence the "G" in the current nomenclature. In North America, all VW/AUDI coolants are produced by Zerex (Ashland Chemical) under authority of BASF. The nonsense that we read about Pentosin being the OEM supplier is simply that - nonsense.
Rundown of VW/Audi coolant standards (norms):
G-11 = TL-774C
G-12 = TL-774D
G-12+ = TL-774F
G-12++= TL-774G
G-13 = TL-774J
If you don't purchase from VW or other suppliers then at the very minimum you should verify that the product you purchase lists the correct VW 'norm' on the bottle for your application. If you want G12++ and the product isn't listed as G12++, at a minimum it should state that it meets VW norm TL-774G. But again, use G13 (TL-774J)
G-13 Die Glückszahl für Ihren Motor!