He's right. Use the Mercedes coolant only. You can use Zerex G-05 too, which is the same color (clearish amber). Out of curiosity, how are you expecting Mercedes to supply you with a new cooling system (free I'm assuming) or engine, considering your car is 12 years old?

Here's a thread that may be helpful:

http://www.peachparts.com/shopforum/showthread.php?threadid=12636