As per the suggestion of Audi...
 

only BP or Shell.

I have to admit, having investigated the system a bit more, it shouldn't make a difference.

Fuel should not be in the intake runners nor on the intake valves. It is FSI, not port injection. Only air should be in those areas. The carbon in the intake runners is presumably from the oil breather system.

Maybe I am missing something, but the fuel doesn't go to the areas in which the carbon is building up.

my bad!