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Just bought a pristine 2008 S550. As is often the case when I buy a Mercedes, I find previous maintenance done wrong. Like the W140 I bought years ago that had the oil filter in the housing wrong, it was crunched up in a ball inside there! this car has green coolant. Carfax shows a coolant change at a local shop three years ago. I called them and asked what coolant they would use if I brought them the car today. Blue, they said. So, it's probable that it was changed since then?
Is there ANY green coolant that's approved for this car? If not, I will drain and fill several times with distilled water, then once clear water, drain again and fill with Zerex G-05 Peach color. It isn't hard to do these cars up properly, I wonder why some people can't.
It's also possible that it was correctly filled with blue coolant, and someone added yellow at a later date. You never know what people have done/will do. Fortunately a coolant flush and refill isn't terribly expensive.