Have a leak down test done by your mechanic
To make it easy on yourself I would have a reputable mechanic do a cylinder leak down test on your vehicle. This will eliminate false ideas as to where the oil is going and pin point where the leakage is most likely taking place. Your car doesn't happen to have an engine driven vacuum pump that operates door locks etc. does it?
"Tell me and I will listen, Teach me and I will learn, Show me and I will accomplish, Involve me and I will succeed."
'84 300SD 256,000 Gold on Brown (Mileage Award)
'86 300E 246,000 Blue on Tan