no it is not that critical. Just dump the old and flush with tap water run for 10m min then dump that lot, then fill with coolant and top up with distilled water. Depending what area you are in just run at 33% coolant to 67%water in cooler areas and 50/50 in qld and hotter areas. I had called up auto barn, and they recommended me Nulon ultra cool, $11 for 5 litres. I think you will find that they ment it will treat 5 liters of water so you will need 2 or 3 of them.