Distinction G11, G12, G12 and G12 + ++

G12 + is already overtaken by G12 ++)

Ff G12 has the advantage that it is suitable as a lifetime filling.
G11 must actually be replaced every 4years.
That G12 ff for older vehicles is not to be appropriate, I think this is a rumor, because
a) it has been officially approved by VW,
b) there is no longer officially the G11 :) and
c) I have 100erte older vehicles so filled, without leading to any damage.

And yes, G12 +, G12 and G11 ++ can be mixed.

No, G12 and G11 are immiscible, then threatens indeed a heat death by clumping.

In addition, the heater core dies on heart attack ...

Hope this has now answered all the questions.

Whether it clumped, you see quickly the filler cap, because there settle brown crumbs.

If it was just a sip ((G12 to G11), I would leave in. If it was a bigger, replace the coolant ...