In a sauce pan, combine sugar, flour, 1/2 teaspoon salt, pineapple juice and eggs. Stir and cook over medium heat until thickened. Remove from heat; add lemon juice and cool to room temperature.
Bring water to a boil, add oil, remaining salt and cook pasta until al dente. Rinse under cold water and drain.
In a large bowl, combine the pasta, egg mixture, mandarin oranges, pineapple and whipped topping. Mix well and refrigerate overnight or until chilled. Before serving add marshmallows and coconut. Toss and serve.