Cook noodles according to package directions and drain. In a bowl, combine the sour cream, milk, and seasoning mix and stir until smooth. In a large skillet, heat the oil over medium heat, add the mushrooms and cook for 5 minutes or until the mushrooms are browned. Remove the mushrooms from the skillet. Add the beef and cook stirring to break up the meat until nicely browned. Drain the beef. Add the mushrooms back to the skillet. Add the noodles,sour cream mixture, and cheese to the skillet and stir until the cheese has melted. Serve immediately.