You could see if there are any certified Nikon repair centers in your area and see if you can have them clean it while you wait.... I don't know if they do this, but would be nice if they would.
I feel if it's showing up by f/8, I would be concerned also.... I've only had to wet clean mine (a few try's) once. Mine had some oil on the sensor, only reason I know was I tried using a brush first, and it smeared whatever it was on the first pass. So it took a few wet cleanings that one round and mines been fine.