Here is the source of oil on most aperture blades and copal shutters. Once you have the retaining ring removed, you will see the pocket where the set screw protrudes through and rests inside the slit to control the blades. This was packed full of 60 year old grease that has partially emulsified and turned to a sludge concrete mixture. The blades were stiff to turn and this is why. Use naphtha on a cotton swab to remove all the old grease. You may need to use a set of picks or a scribe to remove some of it. You can rotate the aperture by putting a #00 screw driver into the slot and rotating the ring. Rotate from full open to closed and clean everything.