When I got my Jetta almost six years ago, I'm not sure if it had G12 in it or not; regardless, I only became aware of G12 about a year ago long after the "green stuff" had found it's way into my engine. The timing belt and water pump were done by the stealership about three years ago, so perhaps it was OK at that point, but I've since replaced several fitings and the radiator, always refilling with green coolant.
Anyway, I've dicovered a couple of things about green coolant in VWs:
1. It won't mix with G12; it stays separate and tends to "float" on top of the G12. When I took off my upper radiator hose (to change the condenser fan) a few weeks ago, bright green coolant came puring out -- even though the stuff in the reservoir was brown-ish. So, a week later, I flushed the systm (or so I thought) and added G12.
Last weekend I broke a heater hose fitting while painting my intake manifold, and more green stuff (so bright it looked brand new) came dripping out of the broken fitting. So, more water, run (with heater valve open), drain, more G12.
When I replaced that fitting this past weekend even more green stuff was coming out, so I took the garden hose and clamped to the inside of the heater delivery hose after disconnecting the heater return line. I ran the hose for 30 or 40 seconds before the green stuff stopped coming out. Now all my coolant is a nice shade of pink...
2. Green coolant "rots" the plastic fittings from the inside out. Here's a pic of that fitting I replaced (the smaller outlet is the one I broke when painting my manifold -- it's almost sealed off!). It was not easy getting it off: