Where is this mysterious FUEL-pressure sending unit that somehow leaks OIL? If you mean the factory OIL-Pressure sender, they are known to leak as they get older. It may be the guilty leaker back there for both leaks. You cannot isolate this device because it grounds to the engine block thru the threads. Well you can but you'd have to engineer a new ground. But these senders in my experience, leak thru the lower perimeter crimp as often as not.
Have you got a working PCV system? If not, then; I would do a CC pressure test
and probably, a compression test.
