If you are sure the howling isnt caused by something else it would probably cost about $20 to change the Differential fluid to Mercedes Spec and it will last a long long time with no headaches.

My contact at the oil analysis place I've used says 75k for dino, 150k for synthetic.

That I'm not 100% sure on. I know that it only howls on deceleration. I don't recall ever hearing it howl on acceleration.

You wouldn't happen to have a part number for the MB fluid?

I'm fairly certain the only cars I've ever changed the diff oil on was my gold 300SD and the 190E. The other three I've never done.

I am wondering if it might be time. Maybe I'll do the 190's with the MB fluid and see if the howling goes away. That would at least tell me if the howling is in the gears or the oil. If it does cure the howling, i'll just use the same spec fluid on the others.

Part numbers for fluids can be found here:

https://bevo.mercedes-benz.com/bevolisten/bevo-sheets-sort1.html

The hypoid gear oil for cars is usually sheet 235.0:
https://bevo.mercedes-benz.com/bevolisten/235.0_en.html

The listed part number is A0009892803 in the 85W-90 weight, although many other oils from different manufacturers also comply.