Add the salt and vegetable oil to 3 quarts boiling water. Add the macaroni and boil rapidly for 10 minutes. Drain and toss with 4 tablespoons butter or margarine.
Saute the mushrooms in 2 tablespoons butter or margarine for about 5 minutes, shaking the pan frequently. Mix the cream, mushroom soup and cheddar cheese together and add to the macaroni. Add the mushrooms, shrimp and crabmeat, which has been cut into bite-size pieces.
Place in a buttered casserole, top with soft bread crumbs which have been tossed with the 1 tablespoon melted butter or margarine. Bake in a 350 degree oven for 25 minutes.