Red Line Synthetic Oil - Gear Oil for Differentials - 75W110 GL-5 Gear Oil
Specifically formulated to provide increased film strength over a -90 oil and help quiet noisy gear sets.

AMSOIL Severe Gear® 75W-110
Wax free formulation for excellent cold weather performance.

X2.

The same thing happened after my full transmission rebuild. They put synthetic oil in and the gears were noisy- you could just hear them in any gear at any rev range. I put dinosaur oil back in and now silent as a mouse.

I have the Liqui Moly in my diff. With regards to the clunk, you'll need to get underneath and check a few things. The flex discs do wear and the bolt holes become oval and not round which gives a kind of clunk when engaging reverse.

Not being an engineer, I have no clue what that chart is telling me. Suspect I am not alone.

Synth must have lots of detergents as the 10K oil drained was pretty dirty.

I'll keep the fresh 80-90w dino in there for a bit before its swapped out again and then fill with the 80-110w Manny suggests. ( which I've never seen and will have to order.)

. Thank you all for your replies.

The chart was to show you that gear oil weight designations are totally separate from engine oil designations, and that a 75W lube is actually as thin as a motor oil. Viscosity and gear protection is important in hypoid gear sets.

Differential whine
 

Sorry folks, use only MB differential oil. There is some kind of additive in it. It's probably about $40 plus per liter.

You don't have to use ONLY MB fluid, but rather MB APPROVED fluid on the MBbevo site. There are plenty of aftermarket fluids on the Bevo site that have been tested and approved by MBZ as factory replacements that would/would have upheld the warranty, so it's all good-to-go with the correct viscosity, flow ratings, additives, etc.

Yes. and I have a pint of the same additive made by GM on my shelf. Limited slip additive. Not required unless its a limited slip w/ clutches.

Might as well use MB engine oil too...

Any good GL5 gear oil will work. These are standard open diffs.

Pretty soon we'll have to use MB approved water to wash our cars too :D

With newer Mercedes I'm a bit of an "approved only" zealot if I'm honest, especially with my computer-controlled Benz that has a 722.6 A/T; but with my W126 I never gave it much thought at all outside of using the right gold coolant.

As I am about to swap the trans fluid with good ol Valvoline Dex High mileage ATF!

Valvoline Maxlife Dex/Merc is a fantastic fluid and is fully synthetic. You'll love it. Drain or drop the pan and remove the drain plug in the torque converter to drain it as well. If you remove the pan, clean the magnet, use a new gasket, don't put RTV on the gasket, and use a torque wrench to tighten the bolts to FSM spec and nothing more.

Mines whining since swapping to redline synthetic... Especially annoying with the manual swap.

Well dont swap back thinking it will reduce the whine! I will try the 80-110W Manny suggests.