[libh8er]

Use the widest possible lens at the smallest aperture to really bring out the crud.

[SWAMPSNIPER]

I looks like some of the D600s are somehow transferring oil to the sensor. Maybe the mirror linkage is being over lubed during assembly. Oil is nasty to deal with, it creeps and spreads.

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.