Id stick with MB 235.0 hypoid oil.

I tried the synthetic in my w140 and I noticed some humming coming from the rear,
had the right amount in there it just didn't seem to be protecting as good.

I switched back to the 235.0 mineral based and it's solid.

Fuchs TITAN Gear Hyp 90 Hypoid Gear Oil GL5 85w-90 was the one I used and it states on their website this is as High-performance gear oil for highly-stressed differentials in commercial vehicles and passenger cars

when you think about it these are heavy beasts and 235.0 mineral hypoid was the original spec the differentials tolerances were designed around.

Theres no combustion going on in the diff so a mineral oil is gonna last just as long as a synthetic... maybe even longer!