I saw this posted over at the Yahoo group that got me thinking:
"Unless you are positive that nothing but Toyota pink coolant has been used in your engine, there is a good chance that there will be head gasket deterioration. When I overheated my 22R-E and blew a head gasket, the mechanic predicted exactly where the failure occurred, and he was right, #4 cylinder, and there was a lot of gasket deterioration around cooling passages, no doubt from the Green antifreeze in the engine for a number of years."
I use the 50/50 green stuff and that's what was in it when I got it almost 2 1/2 years ago.