Mercedes replaced the axle in my 190E 2.6 five-speed twice under warranty due to ring and pinion whine that occurred under "float" conditions such as when you lift to go down a mild downgrade at freeway speed and the input torque equals the drag torque, so there is no load on the gear teeth.

Back then I was traveling between Socal and NoCal several times a year on I-5 and the noise was really annoying.

The axle oil spec for my '88 is SAE 85W-90 GL-5, and I went through hell trying to find any. I finally called Pennzoil that had a product on the approved list and they told me that it was not marketed in North American.

That left 80W-90 GL-5, which is the same as most manufacturers recommend for hypoid axles. It's not "very thick". The absolute viscosity is in the range of SAE 30 to 50 engine oil. Engine oil SAE viscosity ranges from 0 to 60, and gear oils range from 70 to 140, but the absolute viscosity ranges overlap.

As far as ring and pinion whine is considered, changing the drive pinion depth may help, but which way do you go, and setting up an axle is a very precision procedure and the change could make the problem worse.

The best solution on a 25 year old car is probably to turn up the radio.

Ring and pinion whine is usually torque sensitive, so it may only occur under certain driving conditions like what I experienced. A spalled bearing will usually make a constant growl that may be higher pitched as speed increases, but it's not torque senstive.

Duke

The noise comes from the assembly that is worn. take it apart and check it out

The benz gear lube is sold on amazon under the lubro-moly brand.

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2012 BMW X5 (Beef + Granite suspension model)

1995 E300D - The original humming machine (consumed by Flood 2017)
2000 E320 - The evolution (consumed by flood 2017)