Diesel keeps for a long time, just add some Bio-cide and no worries long term.
Diesel ~used to keep for a long time. The unblended diesel still does if you treat it with a biocide so the black algae won't grow.
Here in Progressive Oregon though, most of the diesel sold is 15% biodiesel. Which is just wonderful, all puppies and rainbows except for the fact that the bio component starts oxidizing right away and becomes a noticeable problem after six months.
I have spent a couple days researching the issue and have come away convinced that there isn't an additive on the market that will stabilize biodiesel. It works great if you turn the tanks over every six months but not suitable for a generator fuel.
The alternatives are kerosene, and finding a source of unblended diesel. It is still available as home heating fuel in the Portland area, but in the southern part of the state, only 15% blended fuel is available.
Propane, that will last and stay stable until the tank rusts through and it leaks out.