Preheat oven to 375°F (190°C). Prepare 4 pieces of aluminum foil.
Scrub sweet potatoes well. Pat dry with paper towels. Prick each sweet potato evenly with a fork.
Place each sweet potato on a foil. Drizzle with a teaspoon of vegetable oil and rub the oil all over the sweet potato.
Wrap loosely with aluminum foil, making sure it is sealed. Place on a baking tray and into the oven. Roast for 60 minutes.
Open the foil and continue to roast for another 30 minutes. Insert a knife or fork into the center of each sweet potato. They should feel quite soft. If not, return to oven and continue roasting until sweet potatoes are soft.