Cut the scallions into 3-inch lengths, keeping the thicker white parts and thinner green tops separate.
Warm the oil over medium-low heat in a wok or heavy skillet. Add the scallion whites and cook gently about 5 minutes until pale gold and softened, then add the greens and keep cooking, stirring occasionally, until all the scallions are deep golden brown and crisp, 5-8 minutes more.
Fish out the crisped scallions with a slotted spoon and spread them on a plate. Set the infused oil aside.
Stir the light soy sauce, dark soy sauce, and sugar together in a small bowl until the sugar dissolves.
Boil the noodles in well-salted water until just tender with bite, 3-5 minutes. Drain thoroughly.
Divide the noodles among warm bowls. Spoon about 3 tablespoons of scallion oil over each, drizzle with a quarter of the sauce, and toss vigorously until glossy and evenly coated. Top generously with the crispy scallions and serve immediately.