Does Diesel go off?
I got 9 litres of it sat in a jerry can that I filled up back in October.
If goes off, i'll use it. If not i'll put the can in the car.
I really can't think of a good reason why it should. It will evaporate, giving you some vapours, which will lead to some minisculely higher additive concentrations. I'm assuming you've kept a cap on it, so the vapour-liquid equilibrium will have been reached pretty quickly, and that will have stopped that. I would have thought that the additives are all stable, so there shouldn't be an issue there. Think about fuel dumps - that stuff will be there for longer than that. It can also take 6 weeks just to ship refined product elsewhere in the world - people export diesel to Europe all the time, as there is more of a market there.
It's also millions of years old. The only reason I can think for it to degrade is excessive temperature + pressure. Not the conditions you will have in your garage - like 10,000 psi and 300 degrees F.
Thats what i thought.
had a quick search (might have been an idea to do it first. o well) and it looks like the main thing to be worried about is micro biological growth and a common fungal species called cladisporium resinae or something like it.
It should be ok but I think i'll use it every couple of months and top it back up.