1992 300SD?
 

I just noticed that the w140 from 1992 had a 300SD marking. Does this mean it has a 3.0 motor or the troubled 3.5 motor.

3.5

Yep, 3.5 rod bender. This was about the time the model designation numbers inexplicably became only an indication of position on the model "totem pole" and not of actual engine displacement. Always did wonder why M-B decided to break a well-understood convention in that way after so many years.