Let me just mention in this thread... I have recently learned, the hard way, to be really careful when cleaning the mirror or ground glass in an SLR. About a month ago I tried to clean the parts in my 20-year-old K1000, and eneded up temporarily making things significantly worse. Here's what happened... Since it's an old camera, the little foam that cushions the mirror as it swings up and hits the top of that chamer area was (and is) starting to deteriorate and come off, so some of it had already gotten on part of the mirror. It wasn't much of a problem since I couldn't see that area of the mirror through the viewfinder anyway, but I felt I should try to clean it off anyway. But when I tried to clean it off, I ended up smearing foam -- and the adhesive that is supposed to keep it in place on the top of the chamber -- onto the rest of the mirror. And then since the mirror is so close to the ground glass, I got some on it, too. It was quite nasty. Eventually, after a bit of work, I managed to fix it up so that the smearing is no longer noticable through the viewfinder, but it's still not perfect.
Anyway, the lesson: be very, very careful when cleaning delicate parts of the camera, and make sure you take every precaution you can to avoid making things worse.