Rub the sweet potatoes with the oil and place in an ovenproof baking dish and roast potatoes until tender, about 1 hour, turning once after 30 minutes. When potatoes are cooked, set aside to cool for 10 minutes. Cut the potatoes in half and squeeze the potato pulp into a 3 quart saucepan. Set the flame to medium-low and mash gently until smooth with a potato masher. Add the salt, pepper, nutmeg, sour cream, and butter and fold together until thoroughly combined. Remove from the heat and serve immediately, or keep warm, covered, until ready to serve.