I found kind of an interesting, albeit long, site about penetrating oils at
https://backyardtoolshed.com/best-penetrating-oil/ Maybe PB Blaster Penetrating Oil would be easier and quicker to obtain. You want the Penetrating Oil, as there are now many different products with PB Blaster on the label. Just my opinion, I think Kroil would work better. I also would give it a little more time to work.
Another site about rusted screws, but not on sewing machines.
http://lumberjocks.com/replies/on/4577314 However, I have seen some, even tried some of the suggestions.
Janey - Neat people never make the exciting discoveries I do.