Soeren, I run the E focusing screen in all of my Nikons The E and B screens are the only ones capable of being used on any Nikkor lens as in 6mm through to the 2,000mm lens.
I have, on a couple of occasions, used my camera upside down through a sunroof on a car, using the screen as a basic aid for composition. As these were on dirt roads I certainly got a layer of dust covering the whole camera, including the screen.
At the end of the session (each time) I pulled the screen out and wiped it carefully with my hankerchief. Which by the way are all linen and scrupulously clean, I carry them for this exact purpose.
Once in a studio where I was using the DW-3 waist level finder the stylist sprayed hair spray over the model from behind towards the camera. I was busy elsewhere and when I came back to the camera I flipped the mirror up and couldn't believe the muck that was sitting on top of the glass, as well as the rest of the camera. From memory we pulled it out and washed it with water and a cotton/linen hankerchief.
Mick.