post my $.02 worth. My father had a Ford Taurus that had NEVER had the coolant changed. One time when I was over there nosing around under the hood of it I opened the coolant reservoir cap and noticed that it was very slimy in there. Man, I mean it was awful!! I think he had about 136k miles on it at the time. Within a couple of months the head gasket blew and he purchased another one. The secret, as far as I am concerned, is to change it once a year.
Jim
'85 300D
'95 E320
'97 CRV