Use bananas, not sawdust, that should let you drive at least 200 miles.
kidding aside, try the synthetic but like I said it is no cure all for a noisy differential, flash123 is right it will be louder. I've proved that already
There is however a thickener (MGF?) that comes in a tube, you add it to regular differential oil and it will quieten noisy bearings. Let me know if you want more info I still have some and know where to get it (the boys at
http://www.importpartsspec.com/ sell it)
Actually, my experience has been that with no regular maintenance MB some differentials begin to whine after 200K miles or more. The best remedy is a replacement differential, hopefully a low mileage one. These are entirely too difficult to replace the bearings, properly torquing the pinion nut is very difficult even if you have all the wherewithall to do the job.
I highly suggest finding one from an '85 300SD with its lower ratio. Your car will really run better down the road with only a very slight loss of acceleration. The pre '85 ratio is 3.07 while the '85 used 2.82 and the taller gearing is easier on fuel and the engine will run more quietly at 70+ MPH.