These new diesel engines are much cleaner burning than the engines built only 10 years ago. This is due to improved engine design and also mandated by the US EPA and European Union lean burn requirements. You should not get any significant carbon build up running the engine at any speed so I suggest that you dont worry about it and enjoy using the engine. Keep up with oil changes and replace the heat exchanger zinc and use it. This way you will get fantastic life from the engine.
Stanley