Do you want your viewfinder looking like the picture above? This is what happens to my viewfinder on the X100T after nearly 2 years of normal usage. I don't think any of us wants to, except over time this WILL happen as somehow dust always finds is way into the X100 viewfinder. The X100F is not sealed, and there is no way to open the camera up yourself to clean out the viewfinder. Having owned all previous X100 versions, I have experienced this phenomenon happening to every single X100 I've had. Thankfully, there is an easy way to slow down the accumulation of dust over time (besides not using it).
There are many places to where the dust can get into the viewfinder, but the biggest culprit seems to be the diopter adjustment opening. If you look closely, you'll notice what it really is, is a large gaping mouth next to the viewfinder, free for dust to enter. If we seal that up on the surface however, it'll stop the dust from getting in from that exposed opening. You won't be able to change the diopter adjustments once you do this so make sure you have it on a level you're comfortable with. There are 9 clicks total front to back, and I leave mine on 5.
Sugru is a moldable putty that will harden into rubber of whatever shape you make it into. It's easily removable once hardened and stays in place until you manually remove it. Using a smart portion of the packet, rub it and make a flat sheet to put over the opening. Then slowly massage the surrounding into place. If it's your first time, please practice on something else before you test it on your camera.
Note: The Sugru will become unmoldable in 30 minutes, so figure out what you want to do with the rest of the pack before you start. You can make hooks, raise buttons, add grip, or anything else you want. I used a small part of sugru to add a focusing tab to the X100F. One more thing: DO NOT RUB SUGRU INTO THE OPENING - just lay a flat sheet of sugru on top and massage the sides in, so you can undo it if you want with ease.
edit: I've heard some people saying smugly that one should just use Gaffer's tape to solve this problem.. Don't. Gaff tape will dry up and crust off into powder, which is the last thing you want on that opening.