The rear crankshaft seal ages and oil seeps at the joint between the engine and the gearbox. The telling sign is an oily, smeared bell housing at the bottom: if you see an oil patch there, the rear seal is letting through. The leak starts slowly and can contaminate the clutch or the torque converter as well. The repair is expensive because the seal can only be reached after the gearbox is removed. The part itself is cheap, the labour is not: even in practised hands it is four hours. The fault comes up above 150,000 km, and according to a workshop that works on these engines a fully tight example from this era is hardly ever seen. The front seal of the engine is a separate part and also tends to let through; that one oils up the front of the engine, and its replacement needs the pulley off, not the gearbox.