Preheat the oven to 375 degrees 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. 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 by 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. Garnish with parsley.