Hello all,
Which brand and viscosity of oil is recommended for the rear differential on an MB with ASD system?
-Does anyone know that precisely..??
Thanks.

From the Factory Approved Service List

For limited slip differentials with ASD, 4Matic:

MB Limited Slip Differential Oil Mercedes No. 000 583 09 04

Many thanks for that info.
Will check it out..

While we're on the subject of diff. oils, does anyone notice louder operation with old oil? I want to replace it soon, so what would be a good oil to use? Synthetic or "real?" My diff. is not ASD.

When draining, does the diff. cover have to be removed, or just the lower drain plug?

You don´t have to remove the diff cover, just the lower drain plug. And then the higher one to fill it up.
New oil might reduce loud operation of the diff.
Generally though, if the diff is making whining sound under slight acceleration, the bearings are the culprits.
There should be a Mercedes approved oil that suits yor diff.
-Not sure if it´s the same one as 'dlafever' mentioned, since yours isn´t limited slip.

No whining sound, just sounds like humming. I think it's the diff. The new rear muffler makes a resonating sound, and my mechanic is going to take a look at that. Just thought the oil should be changed, as it hasn't been done in a long time, if ever.

I know it's been said many times, but it bears repeating:

When replacing diff fluid, always remove the FILL plug first. It's more difficult to remove than the drain plug, and you may find yourself in a situation where you've drained the oil, but can't refill it because of a stuck upper fill plug. The upper drain plug is more prone to corrosion, whereas the lower drain plug is bathed in oil.

Great tip, Kestas. Never would've thought of it.