well i had my first D600 for about 30 days..had zero issues with spots or dust or oil..it actually produced some really nice images for me.
here recently i dropped my camera knocking the sensor out of aligment. i had damage coverage on my D600, so i returned it for another one. took 300 some odd photos with it yesterday, zero issues as well. Maybe its because i do a lot of portraiture type of photography to where its hard for me to notice it..but i will test my new body today later, with the white wall test and so on to see what can be visible or not.