Prestone Green is older anti-corrosion technology with silicates that will precipitate out, hence the two year maximum renewal cycle. Zerex G-05 uses HOAT technology similar to Volvo Blue and lasts much longer with no precipitation. Given the cost and difficulty of heater core repair in your car (a big deal!) I would use the best coolant I could find along with distilled water for makeup. "If it ain't broke..." is good advice as long as what you're currently using won't accelerate failure. Unfortunately, Prestone Green will do so.