- oily vapour from the crankcase ( people use so called catch-cans with limited sucess)
- oil leaking down the valve stem - this is the black shiny varnish on the stem
- combustion products that swirl (scavenge) back in to the air intake ports - this is due the the valve timing overlap between exhaust and intake valve opening angles. I am not sure, but i believe the timing is different on EU vs US and the engine in EU is lean burn, so the combustion products are not the same either ( low sulfur fuel is used extensively in Eu and also now in california, but I dont think this is the major problem. Its those combustion particles and the oily vapor that causes the build up over time - it coats the walls of the intake ports 5-8 mm thick, not just the backs of the valves.
I used only top tier Shell 91 Octane from new and it made no difference in both our cars - 2008 S5 and 2010 passat both had to be cleaned at 75K miles. This is using CA gas.