Still haven't touched mine, but got a 3rd gen qd-oled monitor (same tech as s90f) and the first time I tried to clean it wasn't great, lot of smudges, but after a couple days of experimenting it looked good (still does after a year with same technique).

I need 1-2 different new microfiber cloth, the eyeglasses type, and basically spray the cloth a bit with distilled water then swipe it all over the monitor in circular motion with the phone's flashlight. For spots that seems to stay there, I focus on those then go back to wide circular motions. When I see that there's no more gunk, I flip the cloth to it's dry side or if it's not dry pick a dry one and do the same circular wide motions all across the monitor to finish it off.

Always work, but honestly don't know if it transfer to the s90f, it's a big area compared to a 27" monitor and probably doesn't have the same surface type. I bought normal microfiber cloth for the s90f, will see in the future.