I've had the same problem as the OP this summer, I went to my suppliers to pick up a 5l container of aspen 4, i mix aspen 4 with stihl hp super, I ended up buying 2 of them, the store was melting with the heat from the day and the drums had ballooned slightly not a whole lot, when I got home I poured it into My stihl kombi can and low and behold it was shy of the 5l mark on the stihl container. The stihl container is accurate because previous 5l Aspen amounts were correct on the 5l mark. I opened the second canister to top it up, had only just around 4.5 litres in it. I'll probably only have enough to make 4 litres of 2 mix next time. I put it down to evaporation. There's a good quality cap on those aspen containers how the hell does it leak out?