M.B. DOC
Many thanks for your prompt reply. I was afraid your diagnosis might be the reply I would receive!

I am considering changing the oil to Amsoil 85W-140, which I know is a grade too heavy. Do you think it could damage the bearings?
The car runs in a fairly moderate climate. The temperature ranges from 2deg.C in the winter to, say, 33deg in the summer. And there is no sustained high speed touring.
Your opinion will be much appreciated.

Regards, Murray Blackwood.

I've got a noisey Diff in my 300 D and I tried a heavier syn oil. Didn't do a thing ... and I switched back to the proper weight.

190E Diff noise
 

Thanks for your comments Robby. I'll feel happier with the correct grade of synthetic oil in the diff.
Looks as though I'm stuck with the whine for a while.

As a matter of interest, when I first purchased this 190E 2.3 I had trouble accelerating smoothly.
Found that the throttle butterfly-valve return-spring was missing. Fixed the problem by replacing the spring, which is quite awkward to fit.
Take the distributor top off. This will give you sufficient room to get your left arm in behind the intake manifold, to the point where the spring is anchored. The "free" end can then be connected to the bottom lever on the butterfly valve. Total cost: $3.
Regards, Murray Blackwood.