I've run on diesel that was more than 6 years old, no problem. The next batch that went in went bad after 4 months. I had to open the inspection
lid to the tank every other day and suck up all the jelly, that's how fast it grew. The diesel I used was what they call city diesel here, it's part RME. This summer I emptied the tank, cleaned it with alcohol and filled it up with marine
grade diesel. On the city diesel crap, the engine
was doing 450 rpm
on idle (if it ran at all) so I had to tune it up. With the marine
grade diesel it was suddenly going 1100 rpm
, so I had to turn it down again
Whar´t I'm trying to say is that it depends on the diesel. Pour it in a clear container first and inspect it. If it's clean, use it, if not, don't! Don't clean it and use it, it will contaminate your tank, just dispose of it.