the source was chicken fat in the B99, if that makes a difference

Biodiesel is hygroscopic meaning that it absorbs moisture out of the air, it also breaks down over time, unless you happened to add vitamin e to the oil or some other antioxidant its garbage. If their is enough moisture algae will form otherwise the oil runs the risk of going rancid. I wouldn't use any leftovers from last year as anything other than fire starter but if there isn't anything in the barrel and its not obviously rancid i wouldn't worry about it too much. If your in the mood to clean then swish a little methanol around in the barrel and call it a day.