Try using one in a strong wind, or in the rain.
Not everybody is disallowed from working on their plane in its hangar…
And even then, filling oil is normally done before flight – perhaps there are advantages, unappreciated as yet, to flying only in good weather 
< copied from the Carcassonne thread >
I have a cut-off soft drinks bottle as a funnel, whose thread exactly fits my Lycoming filler tube. Drips are contained by screwing on the original bottle top. This bottle lives snugly in another bottle in the cockpit, to contain any oiliness. No weight at all, no messy funnels.
That sounds like a clever and effective arrangement – are you sure you didn’t fly microlights at one time? Cut-off soft drinks bottles can serve many purposes, I use one to hold my pcb drill without breaking the bit, and they also work well for catching flies and other flying nuisances.
Getting it from Sporty’s would cost me $1M in shipping so I emailed Friebe in Germany. It’s obviously the same item, but 3x more expensive in Europe as usual.
If they don’t reply I will harrass some German pilot to help me 
Many thanks for that link.
They used to supply flat packed card funnels if you bought oil by the box.They were long and thin, so it was possible to stick the end of the oil contaianer into the top of the funnel, it worked even in the wind.
My aircraft almost never needs topping up :)
Edit: This is similar
The firm in Germany proved to be really responsive. They even checked it on the real oil bottle, to make sure the thread is right.
