unlike wine diesel does not improve with age. it only turns to crap. so no, throw it away in an environmently friendly fashion, find somewhere that sells red diesel. its half the price of white diesel and many garages sell it as gas oil. or chat up your local farmer for some but only use it in your tractor, do not use it in your road vehicle or you may get a visit from her majestys customs and excise.
Thanks TomTec....we are talking about a farm..believe it or not we have found about 20 gallons tucked away behind the tractor shed. Can dispose safely but wondered if it would muck up the little tractor engine if we used it. It is red diesel and no we wouldn't try using it in a car
being serious now, i wouldnt use it because most old fuel turns to something like thinners, just like petrol goes stale. i wouldnt risk the damage to the engine. but it might be ok for starting fires!