The best way to preserve is to use the MB genuine coolant purchased from the stealer and change every two years. As noted above, a flush may be worth doing. There's a lot of good advice on the forum about changing coolant, etc. Key things to pay attention to: not draining from the drain valve on the radiator, as it can strip out the copper threads. Just leave the drain plug along. Safest way is to drain using the large hose at the bottom of the rad (though a bit messy). Ensure you fill correctly; it's not hard to get an air lock. You should remove the top hose until coolant starts to flow out, run the car and fill the remaining to capacity with the car on an incline, nose up. Note that you'd want to ensure you have the flow to the radiator core open when draining and refilling as it holds a surprising amount of the stuff.
