Until proved otherwise, I'll take Prestone's claims at face value. BUT, Prestone also states on the jug that when their universal extended life antifreeze is used to replace regular antifreeze, to do a full water (I prefer distilled water for this.) flush first or the extended-life properties will be compromised down to the remaining life of the old juice. It would stand to reason that if the Prestone universal were used to top-up conventional antifreeze, the same compromise to its extended-life properties would also occur. In other words, extended life antifreezes - any brand - cannot stop continued deterioration of remaining conventional antifreeze. I suspect that when any OAT long life antifreeze is used to replace another OAT antifreeze, the extended life properties are much less comromised, if at all, unless there're pre-existing cooling system problems. Still, after three or four (take five years with a grain of salt) years, doing a full distilled water flush is arguably a better idea than assuming all will be well with a simple radiator drain and refill.
