This is an M110 engine, basically the same as on a 1977-81 280E or CE, both of which were US cars. It sounds like your leak is from where the cam box joins the head. It is very common and you don't want to mess with it unless it is really bad.

The O2 sensor was probably added when the car was federalized. You can use a generic Bosch single wire sensor as a replacement.

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Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe