E220 cdi avantgarde - automatic diesel. Has covered 105,000 miles. One mechanic says with a higher mileage car I should use 15W40 semi-synthetic oil. It is better for a high mileage vehicle than the one specified - which could damage the engine. Another mechanic friend disagrees. He says I should stick to the recommended Oil - High mileage should not affect the Oil required. Any views on this ??? Also is fuel economy affected by which Oil is used ????