Preheat the oven to 375?F.
Put the water, 1/2 teaspoon of the salt, and the oil in a large, heavy saucepan over high heat and bring to a boil. Add the macaroni and cook, stirring occasionally, until tender, about 8 minutes. Drain and rinse under cool water. Set aside.
In a small, heavy saucepan, melt 2 tablespoons of the butter over medium heat. Add the flour and cook, stirring constantly, for 2 minutes. Slowly add the milk, whisking constantly. Add the remaining 1/2 teaspoon salt and the white pepper and continue whisking until the sauce is smooth and thick enough to coat the back of a wooden spoon, about 8 minutes. Remove the white sauce from the heat and stir in the cheese. Continue stirring until the cheese melts.
Lightly grease a 6 1/2 X 10-inch casserole dish with the remaining 1/2 teaspoon butter. Combine the cheese sauce and macaroni in a large mixing bowl and mix well. Pour into the prepared casserole and bake until lightly golden on top, about 20 minutes. Remove from the oven and serve hot.
Yield: 4 servings