My 1991 300E has been whining for the last 30kmi or so and I have tried all type of fluids and I think I found one that works for me.
The first one I tried was Mobile 1 synthetic 75W90. It did reduce the noise and I left it on for 20kmi.
Then I came up a tip from a Jag mechanic. He said whine could be due to mis-alignment of gears and slightly work gears. He suggested a heavier oil. I wanted to try this so I went to Wal-Mart and got some 85W140 dino oil and out it in. WOW, what a difference in noise reduction.
I was afraid that the dino 85W140 is too thick for winter so I started looking for a heavy synthetic oil. Redline comes to mind and I contacted them about the situation.
They even have a heavier one called ShockProof Heavy (65W250). They did not recommend me using that but said 80W140 should not be too thick. So I bought a couple qrts and out them in. It does miracles, the noice is very reduced now and I can only hear it between 55 - 60 mph now.
To MB purists, please ignore this post as the 80W140 is a heavier oil than MB recommends (SAE 90). But my 300E is 11 years old and it has some miles now so I am willing to try. Since it is a 80W140 synthetic, the low temp viscosity is not too thick.