If it's been sitting that long, especially if it's been in an unsealed container, some of the lighter hydrocarboins will have flashed off. That's no big deal, but I'd make sure to blend it with fresh diesel at least 50/50.
Also, if it's in a large contianer like a drum or barrel I wouldn't stir it or pull from the bottom of the barrel. There will probably be some sediment down there that has settled out naturally.
I have about 15-20 gal. all of it has been in sealed 5 gal cans. couple of buds had from when they used kero heaters. so just mixing with fresh diesel and some lube additive should cover it. sweet thanks!!