While I agree its not a good idea to use the last drop, and pick up moisture - particularly in our frozen northern climes - if the design really needs a 15 liter sump, that should be built in - not taken out of the advertised 60 liter capacity.
I immediately ran into this with my new 3GT bought in October 09. One of the reasons I chose the new Mazda was the big 60 litre tank, but I couldnt put more than 48 litres in. I took this problem to my dealer under warranty - either the tank was not 60 litres (~15 US) or the gauge was faulty...