According to information provided by one of the pros here a while back, the M103 motors got new guides in April of 1989.
Go to the SEARCH facility and key in BLUE SMOKE in the search field. The majority of responses from the pros point to worn guides or valve stem seals. The former is big bucks; the latter not as bad as the head does not have to come off. Stem seals are a diyer job if you have the correct valve spring compression tools and compressed air to hold the valves up on the compression stroke.
Aside from oil loss thru valve guides/stems, you may also be losing it through the valve cover gasket, top front cover, head gasket, oil pan gasket; rear main seal. The loss from these most recently mentioned areas may not be that noticeable, but collectively, add up to more than you might imagine.
These 103 motors are definitely not the most oil tight I've dealt with, especially after you're use to dealing with Asian cars that never use any oil at any mileage.
I do not understand the mileage indication you've provided, but irregardless of where the comma should fall, it would seem your cars cylinder head was serviced not that long ago in terms of mileage. I would be suspicious as Benzmac is about whether the work was done at all. Next I'd wonder about inactivity - drying out of the head.
My 2 cents.
1991 300-SEL - Model 126
M103 - SOHC
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