After much sucess with nulon foaming air intake cleaner on a heavily carboned motobike engine (like absoluely caked onto the inlet valves from running rich), i'm tempted to do it each service (like subaru's). Will likely do a 7.5K interim service and do it then to make sure its all squeaky clean. Seems to be a flaw with DI engines that they carbon up badly. The RS has two sets of injectors to overcome this now (a DI set, and a secondary set at low throttle openings to wash over the valves, and reduce carboning.). I assume most DI engines designed now will start coming with that setup