I have a small tub of Quick-Glo metal polish, it's very effective on old bobbins; I rip off a narrow strip of cotton fabric (like and old sheet), apply some polish and polish them clean and rust free in a couple of minutes (have someone hold the bobbin and you can hold both ends of the cotton strip). It's described as a cleaner and polish in one, and it works. I'm sure there are other polishes that does the same job, but you have search for them; polishes are very different when it comes to which types of metal, dirt, rust and tarnish it works best on.
You can use fine steel wool on them, with and with out soap, rinse in hot water and dry immediately. I don't think there's any advantage to sewing machine oil for this? A good polish gives them more shine and they often look like new after. If there's any pitting it can't be removed, but the rust and grime is.