Okay, an update on the current situation, regarding the Nikon D600.
US advisory, as quoted on Nikonrumors, is only applicable to customers who were part of the class action lawsuit.
UK advisory is that Nikon will clean the sensor, for the lifetime of the D600, should there be oil / dust issues. They may, at their discretion, replace with a similar model, i.e. does not necessarily mean a D610 could be another D600.
As regards the OP, go to Nikon site and follow the necessary steps to request repair. If asked, state it is a warranty repair, shouldn't matter that you brought it used.