First week of February2013. I bought a Nikon D600 & Nikon 24-70mm f2.8 mid range zoom lens and I was on vacation to Hong Kong - Macau - Korea - Taiwan & Japan just came back yesterday. I took many beautiful picture and so far find no issue of dust & oil. I guess some are not so lucky I believe dust & oil is a common thing in all camera sooner or later you need to service your camera it can be clean ask Nikon Service Centre for guidance. You can do it yourself.
you just come out from nowhere is it?
dust issue on D600 is 'self generated' even without changing lens.
Nikon D600 Sensor Dust Time-lapse (pt. 1) - YouTube
This is a fact but Nikon ask all of their service center to tell the same story like you'd said