Scrub potatoes under cool running water and pat completely dry.
Poke each potato 6–8 times with a fork to allow steam to escape.
Rub each potato thoroughly with oil. Sprinkle generously with kosher salt, coating all sides.
Wrap each potato snugly in aluminum foil, sealing completely.
Place foil-wrapped potatoes in the slow cooker in an even layer. Cover and cook on LOW for 7–8 hours or HIGH for 4–5 hours, until fork-tender.
Carefully remove with tongs. Let rest a few minutes, unwrap, slice open, fluff with a fork, and serve hot.
Notes
Use evenly sized russet potatoes for best results. Avoid stacking in the slow cooker for even cooking. Leftovers store well for up to 4 days in the refrigerator.