| What market is this for? C300/C350 in the US only has a dual exhaust. Did you buy it new?
You might want to get the second look at that with an independent dealer/mechanic. I have not heard or felt this rough idle or sputtering you speak of, except maybe at first startup.
I'm sorry about your early woes, but the engine on this car (in my experience) has been great. I did not detect the excessive, or any usage of oil for that matter, during break-in. I changed my own oil at the 13k mark and the oil level was still at the max line.
Hope you sort this out. Owning this car has been the most satisfying of car experiences in a long line of Japanese and German makes.
Regards,
Paolo | 
