In the US oil changes and all other service is free for the first 50,000 miles. At the service intervals MB supplies Mobil One for changes on ALL 2001 models. The problem is, there are no DIESEL models available in the US in the 2001 model year.
From this though, I would expect that Mobil Delvac One would be the correct oil for the diesels. If you can find Mobil Delvac One in your area it would be the best to use. The Delvac oil is especially blended for diesel engine service.
As to the weight, many people have their preferences but in your climate, 15W40 or so, would be an excellent choice. This is the most common weight for diesel engine service with the especially blended oils. This is used in all climates.