Preheat oven to 400?. Wash sweet potatoes and pat dry. Rub a light coating of cooking oil on them and then wrap each in a piece of aluminum foil. Put in a baking pan and bake at 400? for 15 minutes then turn heat down to 350? and cook until tender, about 30 minutes. Test for doneness with a toothpick. Sweet potatoes oiled and wrapped cook well in the coals of a fireplace or camp fire.