Seems too long to me, even with synthetic. 10kmi or once a year would be in line with standard practice around here, providing you don't run the engine cold. Peace of mind would be changing the filter half-way through and topping off. And having an oil analysis done if it's cheap ($30) and convienent - call the trucking companies in your area to find out where they have their oil analyzed. You need to do the oil analysis every change.

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daBenz - 1970 220D