kip's right, oil is cheaper than a head rebuild. I'm not sure about MB but other manufacturers specify that oil burning has to be as bad as 1qt/500 miles before action must be taken. I'm guessing that once it reaches that rate, other factors come into play (e.g., fouling of plugs, converter, O2 sensor, cost of oil, other soot problems) where it makes more sense to rectify the oil burning issue.

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95 E320 Cabriolet, 169K