4th D600, same problem.

I had the same oil problem with my older camera, long story short and to the point, don't let the oil dry up on the surface. It'll be more difficult to wipe off if it hardens(exaggerated a bit) and you could damage your sensor trying to remove a harden oil spot. Just man up and clean it ASAP, keep cleaning it until all the oil in the mirror box is reduced to the point it does not become scattered onto the surface through normal camera operation. I know Nikon QC has disappointed you but you just have to do it. Returning or exchanging the camera will not solve your problem, your just reacquiring a new body LOADED with oil in the mirror box AGAIN.