I personally don't feel that this particular fluid usage requires anything particular special.
I bought a five gallon bucket of 85W90 Conoco hypoid gear oil some time ago, then went to the tractor supply and got a pump to put on top of it.
I've changed the rear axle lube in my 240D about every 100,000 miles since then, that was about 300,000 miles ago, everything's still just fine.
The rear axle, drive shaft, half shafts, etc. were all designed for the gas engine that goes in these cars. You put the 67 HP diesel in front of them and they're destined to run virtually forever.
'01 C Class, Six Speed
'84 Euro 240D, manual, 533K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in