Impossible to say exactly what it is, could be from the paint itself or something coming through the paint (behind the paint film). Fixing is fairly simple; you will need some tinted shellack primer, a disposable paint brush for the primer and the touch-up paint. Wipe down the affected area, allow to dry, then prime the area with the shellack primer (could take 2 coats to seal up the area). After the primer has dried and the 'oily' stuff isn't returning you can touch-up with the original paint.

Double check the exterior of your home for possible water leaks, cracked caulking or peeling paint. Sometimes water leaks can cause this type of problem.