Cook the elbow macaroni in 2 gal/7.57 L of well-salted water until tender, 5 to 7 minutes. Drain the macaroni, toss in the butter, and reserve.
To make the cheese sauce, combine the flour and oil in a saucepan and cook over low heat until it forms a pale paste and the raw flour aroma has dissipated, about 4 minutes. Gradually add the milk, whisking constantly.