When I looked into changing the wife's car back to green stuff, everyone said to FLUSH the old out REAL well. Do not let the 2 mix. I guess Peaak or Prestone even makes an acid flush additive that was suggested to run in between the two. Knowing the care I have always given my wife's Grand Prix, I decide to get rid of the DexCool. One season had it looking like pudding, come spring I am going to flush, flush, flush then go to green stuff. BTW the head and intake gaskets are still intact in her car (or so it seems)