It could be the shutter operating mech itself, which is lubricated at time of manufacture. I wouldn't dare go there. Probably requires some disassembly - arrghh !
https://www.photo.net/forums/topic/287149-shutter-lubrication/
Some good (old) info here. In the past my old dad (a keen amateur) swore by lighter fuel carefully applied by cotton bud. Definitely don't use any lubrication on blades themselves. It will always leave a residue.