I have the smallest one, and have for some time. I use mine less frequently than I should because it’s a complete pain in the arse filling a 5litre paraffin heater tank. Let me explain, as a one handed operation I have to rest it in the top of the can, and then pour the paraffin using two hands to hold a heavy 25litre container. This as you rightly assumed creates the dreaded air lock, and after a small splash of fuel, the funnel remains full and about 10-50% of what I poured is on the floor. Second solution is to pour smaller quantities of fuel into a separate jug, and then using one hand to hold the funnel a few mm proud of the rim of the tank, pour slowly into the funnel. But I find there is so much build up not flowing through that I have to keep stopping, emptying the funnel, and restarting. The process takes forever just to fill 5 litres, and is a complete ball ache. The claimed flow figures (on my unit anyway) are an absolute joke. I will credit it though on removing the solids, and water as promised, however the ‘water’ I discard is saved in a separate container and used to light my bbq charcoal in the summer, so it isn’t JUST trapping water, it’s wasting fuel also. (Just smell what it captures if you don’t believe me). I’m tempted to buy the bigger one like you have but having already blown £27 odd back when I bought the little one I feel slightly cheated that neither the flow rates or ‘fuel in-water out’ claims are true. I’ve never cleaned mine, I finish using and store upside down, before using again when dried out I tap lightly as recommended. 🤷‍♂️