On mine, it appears to be the rear Right hand side oil drain hole in the head gasket that is externally leaking. I'd imagine that the coolant passage near it will be soon to follow and then I'll have to slam a goetze graphite head gasket on it.
You should know if it's oil as it will be black
Fuel is clear and transmission fluid red(if fairly recently serviced)

__________________
97 e300d, 78 300Dt, 95 E300d, 94 E320 estate