Good thought. The 10L tank was 1/2 full when I got it and I added 5L of petrol and 100ML of oil – don’t know how long the original 5L had been sitting in it. So its possible the original 5L may have got a bit richer than the 50:1 mix.
I had been running the boat out in Lyttleton for an hour the night before – but was too late to flush when we got back – as you say, consideration to neighbours! So the flushing was done at about 10AM the next morning.
I’ll have a hunt around the garage for a drum of some kind and maybe head off down the plastic box shop if I can’t find 1.