Good for you, Peter.... but I would say your experience is the exception, not the rule. I've bought many a used (5 year old) car where the coolant was never changed and brown in color, only to fail a bit later from corrosion rot.

I have seen data from Ford Motor Co where they plot corrosion protection versus miles for the green stuff. Right at 30,000 miles it crosses the corrosion threshhold.

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95 E320 Cabriolet, 169K