In a sauce pan, combine sugar, flour, 1/2 teaspoon salt, 1 3/4 cup pineapple juice and eggs. Stir and cook over medium heat until thickened. Remove from heat; add lemon juice and cool to room temperature.
Bring remaining pineapple juice and 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. In another medium bowl, combine mandarin oranges and pineapple. Refrigerate both overnight or until chilled.
Once chilled, mix fruit into pasta mixture, add whipped topping and marshmallows and mix well.