I guess I'm very lucky, I've had my Cam for about 4 months now and I've had no issues with dirt or oil, my sensor is as clean as the day I took it our of the box. What would have made this problem do this to some cam's and some not.
I've been a D7000 user and loved the Cam. I got the D600 and now my D7000 is put up for my grandson. I've been useing it for 3 weeks and I have not had any problems with dust or oil issues. As for me, the D600 will be the Cam for a very long time.