Another area to look at is the "top front cover". If you have access to an MB CD-Rom or an M103 engine manual, reference job 01-212 - Removing, installing top front cover.
In between the "front cover at top" and the top of the timing chain cover is a somewhat U-shaped gasket that's notorious for leaking. I loose a qt. every 1500-2000 miles in my M103 motor and a good bit of it is thru here. A veteran parts guy at a local MB dealership told me that M103 motors can loose alot of oil through this area. A local independent mechanic I know who worked at this same dealership for 8 yrs. had the same opinion.
If you change oil the old fashion way, pull the large plastic shroud from the bottom and clean it thoroughly.
Drive for 500 miles or so and pull it. In my case, I've noticed where it's falling from using this approach.
Your engine is a 1991. In a previous post one of the pros
on this site indicated that guides/seals were upgraded sometime in the late 80's...don't remember the exact date.
Just my 2 cents. Regards.
1991 300-SEL - Model 126
M103 - SOHC
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