unit of measurement

hello friends,

how do you do.please clear my big problem this is liitle for you.

in material master i entered base unit of measurement in liter and order unit in kg and i entrred conversion factor 1000liter = 910 kg.now i do not want to purchase in kg of oil. i want to purchase in liter but when i post purchase order and i put order unit in liter but it is automatically taken in kg. now how can i purchaee oil in liter what to do in material master record.