Combine cornmeal, water and salt in a bowl and set aside. In a large pot, bring 5 cups of water to a boil. Add the oil and stir in the cornmeal. With an electric or hand beater, beat the cornmeal until it starts to thicken (3 to 4 minutes). This will keep the polenta smooth and free of lumps. Cook it over medium heat, stirring constantly with a wooden spoon for 20 to 30 minutes. Turn off heat, cover the pot and shake it a little. This will allow some steam to get under the polenta so it will detach itself from the bottom of the pan, making cleanup easier. Let the polenta stay on the stove for 3 to 5 minutes without stirring it. Pour the polenta out onto a board, or pour it into a large bowl. Let it cool and slice it.