1990 Porsche 911 Question: oil or gearbox leak repair

I would suggest washing down the engine (steam cleaning it if necessary) to pin point the leaks. Ensure the engine breathing is OK, ensure the PCV valve is clear and functioning correctly or any blow by will build up crank case pressure and cause oil leaks from camshaft seals, valve cover gaskets and the crank rear seal.The rear crank seal is a one piece lip seal not a rope seal like older American cars or a "scroll seal' like older British cars.
With a clean engine the engine can be run to determine where the leaks are coming from. Florescent dye can be added to the to either the engine oil or gearbox oil (but only to one), and when inspected under black light the dye may more clearly show where the source of the oil leak is coming from.
To replace the crank rear seal the flywheel has to be removed, you will be replacing the clutch any way.
I love old cars, owned and work on a lot of old cars, (French and British). You may want to pull the engine and just reseal every thing if you are doing the work yourself. Timing belt replacement is important on these cars as well.