I've been considering a Lexus IS250, and my internet research has highlighted problems with carbon build-up on the intake valves of the direct injection V6. I assume as the problem is because the valves aren't exposed to fuel, the grade of fuel used is irrelevant, which confuses me as to why the internet seems to blame "lower quality" fuel in the USA (where most of the reports seem to originate).

Is it a known problem in the UK on this engine? Does the type of oil used have an effect? As an engine ages and naturally uses more oil, does the problem get worse? And finally (!) what of hydrogen cleaning services I've seen mentioned on UK forums?