back differential leaks

["Maksim Libenson" <max@voras.lt>]

Hello,

I have problem with rear differential. I can see some leaks
at the connection of differential and one of the hubs.
When I bought my '88 90q six month ago it was OK
Three month ago I took my Audi to service center and
during the check-up they added some oil to the rear differential.
Now I think that they probably used wrong oil. I think
they added same kind of oil to the gear box and rear diff.
Is it OK, o is it different? May be it is good idea to change
oil in rear diff? What kind of oil I should use for the rear
diff, and what kind of oil for gear box? I suppose that 
front and middle differentials interconnected with the gear box
and use same oil, is it right?

It is my first post to the list, I would appreciate your answers.

Max Libenson
Klaipeda, Lithuania
max@voras.lt
'88 Audi 90 Q