'Genuine' oil will probably be whatever brand of the correct spec MB are getting the best deal on, supplied in a bottle that looks suspiciously identical to Febi stuff (08971 in this case) save for the sticker and probably around a tenner per litre based on a quick look at
TRANSMISSION OI - E Class 1985 - 1994 E250D - Mercedes-Benz Parts, Inchcape
Any good quality dexron ll or dexron lll will be fine in these old slush boxes, a dexron lll should/ might shift more smoothly at low temps. If you look in the back of your owners handbook there'll be a list of all the different fluids, fill and change capacities and an MB spec like 229.1 for engine oil, possibly a choice of several for autoboxes like 236.1, 236.6
You can look them up here for a list of some of the oils that meet the specs
Mercedes-Benz Specifications for Operating Fluids: Engine Oil, Gear Oil, ATF, Coolant, Brakefluid
Something like Fuchs Titan ATF 4000 meets/exceeds the specs and would be a good choice if you want to mail order from somewhere like Opie oils. Fuchs ATF 3000 is a dexron ll and a bit cheaper
Fuchs Titan syn 10w40 is a damn good semi synthetic engine at sensible money but like their ATF not something you often find stocked at a local motor factors
If you'd rather buy locally then Shell Helix HX7 10w40 is easy to find and not too expensive from Eurocraparts during one of their "sales" although often cheaper mailordered from their carparts4less alias
Note that in many cases 'better' oils aren't actually better, they're formulated differently for more modern engine designs that need very different things from engine oils or even more about pandering to extended service intervals that car manufacturers love because it appeals to fleet buyers and renters
