Chevron Techron is one of the all time greats for cleaning up dirty system, so is STP Fuel Injector Cleaner and for extreme conditions, Gumout Regane. Techron is now added to premium gas here in India at the pumps. For diesel I use Elf DALF which I have found to be the most effective as it removes carbon deposits as well. DALF is recommended every 10000km, its quite abrasive and I have seen it wipe out my old hoses etc. however it works pretty good on extremely clogged engines.