Soak or flush it thoroughly with solvent. Light trace of oil on the pivots, trace of grease where springs move against the case or other parts and where parts slide/move against each other. Fully disassemble if you dare. The linked manual has some basic instructions for servicing shutters covered in that manual also. Soak rusted parts in cider vinegar for 15 to 30 minutes, wipe any rust residue off, wash the parts and dry thoroughly then immediately coat with oil or graphite (shutter/aperture blades) or it will rust back within a very short period of time.