[doru]

Actually BMW issued a statement or bulletin a few years ago concerning cleaning of fuel system - it started with the N54 engine, when they had a bunch of failed HPFP.
To make it kosher, they took the Chevron fuel system cleaner and and bottled it in a similar bottle as the Chevron, only it was grey/silver in color and re-labeled BMW fuel system cleaner, and in small letters on the back it was marked Chevron made for BMW. I assume it's the same product. Also, there was a recommended mileage interval, can't recall from the top of my head. Shell V-power has the stuff incorporated in the gasoline they sell.
I used the Techron & also the BMW product. Both had same result. On normally aspirated engines, one can feel the change, but it takes almost a tank to visualize (you see the fuel consumption improvement). The change is slow, so you can't tell, but change for better it is. One way is to record the engine sound before, then when the tank is almost empty another sound recording. You will notice.
On the turbo engines, it will clean the fuel system, but i will do squat to clean the valves. The fuel is not sprayed behind the valves, so carbon buildup is guaranteed, less on N55 compared to N54.
Walnut blasting on N54 if done professional by a shop is in the 400 bux range, money well spent. If you know what you're doing, one can do it at home, but you need to buy some tools and the media. Again careful, because you can plug up solid the engine if not 100% sure where TDC is - you have to turn the engine for every cylinder you service, and it has to be closed.