In a saucepan, combine sugar, flour, 1/2 tsp 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 boil, add oil, remaining salt and cook pasta 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.