As promised here is my update regarding my D600 oil-spot issue.

My D600 back from Nikon Service after a week (2nd service) and the invoice repair description reads:

Service Rank: CK

Repair: CLN Image Sensor.

There are no oil-spots now and I hope it stays this way. I will let you know after I use it for while

Nikon cleaned the sensor, but I can see some dust wen looking through the Viewfinder, how to clean it?

Ed.