The more I thought about this the more serious a problem I considered it might be. I would pull the oil out the dip stick tube and put a litre or two into a calabrated metal container. This I might do at the half way point between normal oil changes. You might get by with a measuring stick as well.
Raise the container to four hundred degrees. I would not use an open flame but cautiously use something like an old electric hotplate. Outside well clear of houses etc of course. I am really concerned about a safety hazzard here. It might be better for the test to use about a pint of oil for example. Keep the oil about 400 degrees for about 1/2 hour. Read your level after it cools again. Calculate the amount percentagewise of contaminates boiled off.
Talk to mercedes at higher than dealer level. Talk to lubrication oil supply people as well. That is if the level drops off somewhat.
You cannot expect really long life of the engines if there is too much in the oil. The experts will know the tollerance allowance. Water in the oil will damage bearings as far as I know. Developing sludge might even oil starve something in the oil circuit as well.
If for whatever reason the charactaristic of these engines cannot be defeated by modification. I would consider extracting all the oil periodically between oil changes I could get out the dip stick tube. Heat it up to boil off the water etc. and pour it back into the engine. Filtering it of course.
So much depends on what the lubrication experts have to say. Certainly a pain in the posterior. I would much rather just try a road tube crankcase ventallation system attempt first. Or if these engines have an oil cooler investigate that and the operating oil temperature. Any of these methods might extend your engine life signifigantly. No I do not think the sky is falling. But moisture especially in the oil is a known bad condition.
I again am not an expert in any way. Even hope I am wrong in my beliefs in this area. Or the problem is not as great as I suspect.