Brake or carb cleaner on the outside. Then to remove some oxidation, find access to a polishing wheel and some mothers mag aluminum polish. If you cant find a wheel, depending on the level of oxidation, Aluminum or brass wire brush followed by green scotch brite followed by white scotch brite then some polish. You can skip the first steps depending on teh condition. ITs not easy to get in thoes little places and will take some time. hth