This discussion proves one thing for certain: we are both semi-fanatical MB owners.
Per the manual, severe operating conditions are defined as:
(1) More than 50% short distance driving (less than 5 miles per trip) combined with engine operating tempatures of less than 176 degrees F, especially with low outside tempatures, and
(2) More than 50% operation in lower gears combined with high engine load (mountain driving, poor roads, etc.)
It is difficult to know the answer to the OC interval question.
Unless we have some direct way of measuring the lubricating abilities of our oil, we must rely on external evidence to make a reasoned judgement about when it is unable to lubricate any longer. Therefore, we either have to (a) trust our own judgement, (2) trust other peoples' judgement, or (3) a combination of the two.
I finally decided to trust those with the highest level of expertise, which I feel are the designers, engineers, and builders of the car. I am absolutely certain they ran exacting tests to determine the best OC intervals for the car to live a full life. They know more about oil than I will ever know, and I don't think it prudent that I trust my own judgement over their's in this matter.
Let me ask you this hypothetical question: if you went to Germany, and you found the engineer that designed your engine, and he advised you, based on the numerous tests that MB did, that you should change your oil every 7k miles, would you believe him?
Since he is the foremost expert on that engine in the world, I would.
And I do.