I am trying to replace the focusing screen in my Nikon FM - It got some of the sticky black goo on it from the surrounding seals. I know the FM does not have user replaceable screen but my camera tech said he would be able to do it if I find the part. I know the FE and FM screens are the same but I can seem to locate any Type K screens only Type E and Type B. What am I changing if I got with one of these other screens. Will I have to compensate? Will it be harder to focus? I mostly shoot with a 50 f1.8 and a 200 f.4.