Shulchan Aruch in Hilchos Chanukah (OC 677:4) cites a din from the Sheiltos that after Chanukah is over, the leftover oil from your menorah that did not completely burn needs to be gathered together and burned in one big fire. The reason, the Mechaber states, is because that leftover oil is "huktzah l'mitzvaso.” Or, since it was set aside for mitzvah use, it then becomes assur b'hanah (forbidden to benefit from), so the only thing to do with it is to burn it off. Maran then concludes with: "if that leftover Chanukah oil became mixed with regular (non-Chanukah) oil, and there isn't 60 times (shishim) the amount of regular oil to mevatel the leftover Chanukah oil, there are those who say that you cannot add the needed amount of regular oil to mevatel the Chanukah oil."...