If you have an assistant you can hold the
transmission end of the cable vertically and just keep spraying with penetrating
oil while the other person works the shifter. A little bit of fluid will get in there. Often it’s just the ends that are corroded. Also check for anywhere the cable might be bent a bit too much when routed through, and is causing internal friction.
Good thing it’s not your tranny. So in the worst case you’ll still be able to shift gears it will just be a little bit Neanderthal