Re: Toyota Prius 2nd Gen. - converted to LPG bybrid I have left gasoline in 5 gal tanks for years w/o a problem. However if you store gasoline for a long period you should add the suppl\ement to keep it fresh. ( I forgot the name of that stuff) One way to check it is to take a small amount in a safe container and light it off. Not technically precise but it works. Incidentally Andrei, That was a great posting and a neat job.