I used to go too all that problem, of using a 1 litre mixing bottle
and topping it..... But I can't be bothered fiddling around.

So I now have a seperate plastic petrol container (5 litres), when it
gets nearly empty, I go and fill it back up with 4 litres of petrol,
then pour in 100ml of oil (assuming 40:1 mix), as most oil bottles
have a see throu measuring gauge. Swish it around and away you go.

I have no problems keeping it from one year to the next (over winter)