I think I would talk to them nicely about using non-MB spec coolant. They may feel it is ok. I do not. The price difference for the green is so small. What other MB recommendations do they ignore? Your cooling system will probably run a long time on green. It will probably run longer on the factory coolant. Then again, you should change it every two years anyway. You can buy a lot of antifreeze for the price of one radiator.