20. The oil that remains in the menorah after Chanukah, and similarly, the
wicks, should be burned. Since they have been set aside for use in a
mitzvah ("hukztu le'mitzvoson") (1), it is forbidden to use them for
another purpose unless, at the outset, one made a stipulation that whatever
remains will not be considered to be set aside for the mitzvah.
(1) The oil and wicks are only considered to be set aside for the mitzvah
and prohibited for other purposes, if they have actually been used for the
mitzvah (that is, they have been lit in the menorah as Chanuka lights).
Simply designating or purchasing a bottle of oil with the intent to use it
for Chanuka lights, does not prohibit one from using it for other purposes
(See Mishna Berura 673:21).