Diesel ha a small molecular strucure and is very penetrating.
A gas tank or fittings that won't leak will leak diesel.
Can't do it any more since the government decided it's dangerous to your health but a mixture of creasote and diesel is the best penetrating oil that money can buy.
I've had scaffold clamps that were in a 5 gallon buket that got filled with rain water and rusted up so bad that you would break the bolts off trying to loosen them, soaked for a few days in diesel and creasote and be able to run the nuts off by hand.
Anything that works, the nanny state bans as dangerous!
A guy once said that tincture of Iodine loosened 50 year-old rusted fasteners; I've been tempted to try it.