I have a lot of trouble here with pine pollen, if it sits on the sensor for a while it is hard to remove. My Rocket blower handles most problems if they aren't sticky.
Cleaning a sensor isn't difficult but many people don't want to tackle it. At 40 to 50 bucks a pop a lot of them just put up with a dirty sensor. That's an outrageous fee BTW, for a 10 minute job.
I was surprised that a modern Nikon would have a dust problem because the sensor vibration is supposed to take care of that. But if it’s oil, that’s different, not to mention nasty. Some of Nikon’s AIS lenses from the 80s had the problem of oil from the aperture blades leaking out and depositing onto the glass. But this is the first time I am hearing of oil on the sensor.
Has this problem been reported on the DPreview forums ? If it’s really widespread then Ken Rockwell will address it at some point too.