In case this is an E320 then it's the 3.2 V-6 and is supposed to be using Mobil-1 15/50 or 0W40. My own '01 C320 with the same engine at 33000 miles has no measurable oil consumption and I change it every 5000 miles. It has always used Mobil-1 0W40 which is factory fill. The brown muck on the oil cap is just condensation and is normal particularly in cold weather. All cars do this.

If this car has been using non-synthetic oil and has not been changed when called for, this might be the famous sludging problem that caused so much engine work in early MLs. Assuming your car has a dipstick (mine doesn't), you should be keeping track of the oil level that way and certainly not waiting for the low oil light to illuminate.

And don't necessarily accept without question what the dealer says. Is this a Starmark warranty?

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Roger E.