Is this a new development or has the car always done it since you got it?
I have spilled oil on my exhaust manifold when refilling or changing oil, and have had valve cover/oil fill cap leakage dribble down onto the manifold and smoke. The air intake for the interior air circulation system is just in front of the base of the windshield, and fumes from these sources (oil burning on the manifold) can easily get into the passenger compartment. Another source of fumes is the crankcase ventilation system. If this is disconnected and you have significant blow by, you will also smell fumes from this source in the interior.
Hope this helps, Jim
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)