'93 A80q 2.3 gear/differential leakage

my beloved and, up to now extremely well behaved, 1993 Audi 80 quattro Avant, with the "famous" 5 cylinder engine, 2.3l has just started
to lose gear oil. Since the gearbox is directly connected to the torsen differential, I want to fix this as soon as possible, because a broken differential would be the death penalty for this lovely car.
I noticed it during a routine check. The middle differential is quite stained with fried oil (since it sits directly next to the catalyzer(sp?)) and I noticed one large fresh drop of oil. If I do with the car what it has been built for (i.e. take some kilometers of curved and slopy (sp?) streets at reasonable speeds) and then stop, there is a clear smell of oil being fried and some times one can see small puffs of smoke coming from the vicinity of the differential/catalyser.
It has 300'000 km on its counter and I would like to know two or three things.
1) what is the most probable cause of this leakage and how much will it probably cost to fix this (parts and amount of work)? 2) where do I fill the new gear oil into the gearbox? I could not find anything that looks like something reasonable. 3) do I need this original oil I get at my local Audi-centre (17 EUR per liter), or does any SAE 75W150 do? Thanks for any hint before I command this car into the hands of some unchristly mechanic.
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