
I suggest the following:
Under the keep rank star is the yellow progress line - when a keep hits rank 0 the yellow line will determine how many oils you have left to use (if its almost full = 3-4 oils ) and once that yellow line is gone, no more oil until you get resources, this will also give an option to defenders, either work for full rank 1 and repair doors or use oils, because regardless on what people say, oil dropped at the right moment can have a profound change on the siege at hand, I've seen the damn oil wipe almost entire wb's.