I think you are right, I see no wires to my reservior either so unless it is tied in somewhere else (not likely I would think) the oil level light just indicates engine oil. You have to forgive me, when I start diagnosing 500 miles from home with the wife & kids (+ 1 future son-in-law) depending on me to get them home, my mind spins off in various directions trying to make sense of all the symptoms. Then I add to that it is a MB and I can come up with various weird scenarios that make sense at the time.

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1999 E320 (gave to daughter) 250K
1994 F150 (may be the end of its road) 240K
1989 560 SL (Reassembly after paint job) 160K
1986 560 SL (deceased., gave it's life protecting my wife)
1988 300 TE (departed)
1994 945T (still running strong with an upgraded cam) 349K
1986 242Ti (deceased)
1968 GT500KR (under restoration)