I have just purchased an SLK 230 with 120K on the clock. The car has been meticulously serviced by MB of Salisbury with oil changes approx. every 10 to 11K using Magnatec SAE 10/40W semi-synth (this is MB's recommended grade of oil for this car). Due to its mileage I was thinking of continuing to a fixed service period of every 6K miles but moving to a 5/40W Fully Synthetic oil (using MB spec 229.1 oils). Is this a good idea based upon current engine mileage. The engine currently appears to use very little oil between services and runs extremely well.