Combine the sugar and cornstarch, add the lemon juice, oleo, and milk Put in pan over low heat. Beat 3 egg yolks in small bowl. Add a little hot mixture from pan into the egg yolks, mix well and add all to pan. Cook until thick, remove from heat, and add 1 cup sour cream and stir. Pour into a graham cracker crust. Refrigerate overnight or several hours. Spread with Cool Whip.