Hilchos Chanukah - Laws of Chanukah
One who does not have enough oil should place the required amount in one holder and divide the remaining oil amongst the other holders. If he would put equal amounts in each holder, he may not have the required amount in even one holder.
--Magen Avrohom
One must put enough oil in the Menorah for the flames to burn at least 30 minutes after the stars have appeared. If they don't burn this long, the Mitzvah has not been fulfilled and he must put the requisite amount of oil in and relight it, without a Brocha.
--Shulchan Aruch 672:1, Mishna Berurah 5
