Generally, for OM647 engines, you'll want a low-sulfur diesel engine oil meeting the following specifications:
* MB 229.1, 229.31, or 229.51: These are Mercedes-Benz specifications. 229.51 is typically the latest and best option if your engine is compatible.
* CJ-4 or CI-4 Plus: These are API (American Petroleum Institute) specifications. CJ-4 is generally preferred for newer engines.
The viscosity (like 15W-40, 5W-40, etc.) will also be specified in your owner's manual. This depends on your climate; thicker oils (higher second number) are generally better for hotter climates, while thinner oils are better for colder climates.
Do not rely solely on this information. Check your owner's manual for the precise oil specifications recommended by the manufacturer. Using the wrong oil can severely damage your engine.