When right clicking on a barrel to either fill up or empty lamps with a stack of multiple lamps in hand, only one empty (or filled) lamp will be consumed and the liquid properly updated in the barrel; but the whole stack of lamps is obtained in return.

e.g.
16 empty lamps, clicking on a barrel with 1000mB of lamp fuel -> 15 empty lamps, 16 filled lamps, 750mB of lamp fuel in the barrel.