For particles of dust- remove lense,hold camera upside down and use a blow brush or compressed air. For grease or smears use a good lense cleaning solution lightly sprayed onto a lense cloth (prefrebly silk). If your mz uses a black foam damper running along the front of the focusing screen (where the front of the mirror hits when open) Avoid disturbing it as I have found this degrades on older models and can cause a mess inside the camera.
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