I also recently bought a refurbished D600. I've only took about 800-1000 shots so far with only 1 dust spot showing up in one of my shots. Because of the dust issue, I've bought a Visible Dust 7x loupe & Arctic Butterfly. When the loupe arrived I checked my sensor with it but couldn't see any evidence of oil etc, just a couple of dust particles which the Arctic Butterfly removed. Touch wood, but it looks like Imay have a good one!!