I just bought a used Schumacher and included in the package was old nitro fuel. This has been in the bottle for approx 2 years. question, How old is too old to use for the fuel?
What kind of bottle is it in? If its in a sealed container it may be ok. But if it has ever been opened chances are it not any good. Nitro will collect water/condensation.
I would use it as weed killer. You might spend days wondering why you can't get the engine tuned right just because your 20% nitro has degraded to 12%.
Once it's lost it's 'factory seal' and then sat for 2 years, I wouldn't trust it any more. It won't damage your engine, but you can probably expect poor performance and nighmare tuning issues.
I've been there, and fuel is always the last thing you suspect.