I always put any old or unknown fuel to one side and use it to clean chains etc, or bung it in the lawn mower.

I would just buy more fuel, mix it with 2 stroke oil at a slightly higher rate than normal and then add some of your original fuel at about 25% of the total. Might take a while to use it all but is a pretty safe way.

we're talking about something thats worth say 30 quid for a 25 litre jerry can.. give it to someone for a tenner for thier car.. job done.. i always put the oil in the can even if i dont put petrol in staright away so that i know that thiers oil in it always..

In order to avoid this situation I have two things that I do. One is that I have "mixing tags" that you can attach to you fuel jug and you can switch the tag to show if you have mixed it or not. The second thing I do is that when I have a 2 cycle oil that does not have enough dye in it, I add some. I bought a little bottle of blue dye that is used for dying candles - make sure you get an oil based dye and not a soy based one. It takes about 5 drops of oil per gallon to tint the fuel enough to show vividly that the fuel is mixed. I got the dye from a craft seller on eBay - but any craft shop that deals with candles will have it.

use a test paper such as a white paper towel, put drops of known 2 cycle mix, straight gas, and your mystery gas, let the fuel evaporate for abought 1/2 a day and compare the residue left, the 2 cycle mix will leave an oily spot.

I had a mix I wasn't sure about once, however it did leave a residue on the hotel balcony

In order to avoid this situation I have two things that I do. One is that I have "mixing tags" that you can attach to you fuel jug and you can switch the tag to show if you have mixed it or not. The second thing I do is that when I have a 2 cycle oil that does not have enough dye in it, I add some. I bought a little bottle of blue dye that is used for dying candles - make sure you get an oil based dye and not a soy based one. It takes about 5 drops of oil per gallon to tint the fuel enough to show vividly that the fuel is mixed. I got the dye from a craft seller on eBay - but any craft shop that deals with candles will have it.

OK this is going to seem insane (like much of what I say) but I was wondering what would happen if you dropped a few drops of candle scent into your premix. You know to sort of... freshen up the line at the section start.