Your best bet will be to use the SEARCH facility here to get a better idea of what could be going on. I'll review some of what's been discussed at this site many times before.
Your car is a '91...not real sure if your model has the single cam(M103 motor) or the dual cam(M104).
The M103 motors are notorious for oil usage. In April of '89, MB supposedly began fitting the M103 motors with improved valve guides/seals. You mention seeing no smoke. Benzmac has mentioned that if the oil is getting by the exhaust seal/guide, you may not notice burning. Randy Durrance, another pro who used to respond here mentioned that M103 motors guise oil consumption fairly well.
I'd be inclined to agree. My 103 motor uses a qt. every 1200-1500 miles and my spark plugs are bone dry. No smoking whatsoever. I too have a little seepage from the U-shaped gasket at the front of the motor, but that would hardly account for the oil loss you and I experience.
From what I've read here and at one other MB web site, valve guides, valve seals, and leaking head gaskets seem to be to primary culprits.
1991 300-SEL - Model 126
M103 - SOHC
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