Have you let Nikon know about your problem/issue?

If mounting heavier lenses on the D600 is the cause of the problem, then as long as only light lenses are used there should be no problem. However, I know of a couple of D600 with significant oil/dust/whatever problem (i.e. "the problem") where nothing but light lenses have been used. Nothing heavier than the kit lens was ever mounted (and a blower doesn't remove them).

If the plastic is somehow the cause of the problem, can you explain how that would technically happen? Putting aside the coincidence (or not) of the D7000 and D600 to have more of a problem than other cameras, how would the plastic cause it?