Last year when I changed the differential fluid in my '83 300CD-T, I used Red Line (synthetic). I've used several of their other products so I was already fairly convinced of the quality of their products.
Without looking at the bottle, I don't recall the viscosity. Nice part was I called them and told them what the owner's manual said and then I got passed to a fellow who seemed to be an expert in these lubrication matters. He asked me the model and year and told me what to use. So far, I've been pleased with the results (I suppose I'd only have a negative opinion if something bad ensued subsequent to the new fluid being put in). For now, the rear end seems to function as I'd expect.
Two other things:
It is an excellent idea to clean the vent on top of the differential while you're in there. Also, make certain that the fill plug will unscrew before you go draining out the old fluid...