2005 Touareg - 3.2L BMX
Don't have an FSI, but do have two TSI engines that have similar carbon build up issues. When it gets bad you start getting misfire codes. One way to reduce the problem is clean oil. I change at half the recommended duration. The 2.0lL uses just under 4qts and takes no time to change as filter is on top. Spend more time removing and replacing bottom pan than the change. Never tried additives as think they are useless related to the problem.The fuel never touches the intake manifold, intake ports or the top of the intake valves.
Good quality fuel can keep the combustion chamber and exhaust valves clean in that engine.
Video Link: https://www.youtube.com/watch?v=d021ePu_3wo