I'm going thru this and have considerable experience. My car's been on the most sophisticated diagnostic equipment in the land, operated by the best tech in the region (per MBZ, and the owner of a 500E) as well as the regional factory tech. My car, at like 1,400 RPM retards one cylinder, and one cylinder only. Further, upon acceleration it feels a bit peaky, not very linear. It used to idle very rough at first in the a.m. (sure sign of carbon deposits)but I've since treated it with BG's "44K"...considered by many to be the best, far better than Techron. However, it's tough to get. And it's best when injected directly into the fuel rail.
My car's first 4-5 years were spent, believe it or not, as a private limosine! It went about 4k/year, and they were easy miles. But they were perfect for collecting carbon, which I am now dealing with.
I'm gonna do the direct-inject BG program next, and will advise on my results.