I've never heard of that bolt requiring an o-ring, maybe the later 103 engine's in the 190 called for it, but I would imagine the black MB sealant would do just fine.
To access that bolt, you'll need to remove the fan shroud and fan clutch as well. The bolt is right behind the TDC sensor, and if you see oil in that area, that's definitely the spot it's leaking from.
'01 E430, Sport 72,000 mi
'98 C280, 126,500 mi