Dont know if this is any assurance to anyone with last years fuel but I will add my twopence anyhoo.
Your post reminded me that I filled and mixed a 5 litre container of E5 last year in April.. mixed 50:1 with quicksilver two stroke oil for good measure .. for my Aux outboard.
There was also half an internal tank in the outboard from the same time. No snake oil whatsoever.. Its 11 months old ..plenty time to bubble and brew if it wanted too
I set it up this morning to get it ready for the new season. Took 7 pulls to start which was not surprising as I run the carb dry after use.
Its been kept all winter in a damp outside locker but with fuel tank firmly closed.
Im pleased but not surprised its working perfect.Not even any smokier than normal. My plan is to use the rest of the fuel over the next couple of days camping up the loch..so I have a new batch for the season.
I also have 50 litres in the tanks of my larger inflatable.. again E5 but bought in September.. so it will definitely be fine therefore Im not starting that outboard up yet Its for the 4stroke EFI.
I gave up the snake oil long time ago ..Guinness and Brewdog mix being my preferred tipple now.