D600 3 questions
1. Will the Oil on the sensor cause any long term effects?
2. How many times will Nikon clean a sensor under warranty
3. Do you have to pay shipping?
1. It depends on what type of oil it is and how it's cleaned. Best ask Nikon Service directly.
2. Probably as many times as you deem fit. But again, best to ask your local Nikon Service.
3. Probably, but ask etc...