Absolutely no need to get nasty. Honestly.
I guess we may need to define what we mean by "backward compatible." The way I mean it, a newer coolant is backwards-compatible when it can be mixed with an older coolant that was originally filled in the vehicle, or that's currently in the vehicle.
I know what ETKA is, though I've never used it. So I don't know what they say, or how they say it.
I'm just saying that VW's and Audi's own official advice (see the TSBs that
GTI's and I linked to) has consistently been that the newer coolants (G12+ and newer) are all backwards-compatible with older coolants.
I do see the bit in the VW TSB where it says this is for "nformation only" for warranty use, and in the Audi TSB where it says it's "not applicable to any Audi Warranty." I'm not sure what that's referring to, but I don't think any of us are worried about warranty work for Mk5's at this point. And I really don't think VW/Audi would officially state that mixing is OK, if it wasn't...
I just called my local VW part department (Mike Maroone VW in Colorado Springs). The parts guy confirmed that G13 and now G12evo are backwards-compatible with past coolants. In fact, he said that those are the
only two types of VW/Audi coolants they currently sell. He added that these days, for warranty work, they only use G13 for 2015 diesel engines (for whatever reason). For everything else, they only sell G12evo now.
I've got about half a bottle of the VW G 012 A8FA4 on the garage shelf that I've used in (and VW dealers have been putting into) our two Mk5's for over a decade now...
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...as well as a full bottle of G13. I think I'm just fine with those. If a shop or dealer puts in G12evo now, I think that would be fine too.