yeah, they aren't 'seeping' anything. It's either damaged threads or seal interface, or fuel dripped or spilled on the outside of the jug.
If a person isn't already tied to NATO-spec MFC / Jerry can
carriers, I would suggest getting the newish Scepter 'civilian' jugs. They look about the same, have a wider bottom for better stability and are about 2/3 the wall thickness, by weight. And they cost $24 brand new.
And I'm using ~7' of 1/2" ID clear PVC hose and a copper 'Super Siphon' fitting for transferring fuel. No standing around holding 50# and trying to use crappy CARB / EPA nozzles. Just set the jug / can so the bottom is higher than the filler neck on the vehicle, insert the open end of the hose in the tank, insert the SuperSiphon in the jug / can and give it 4-5 vertical jerks and the fuel starts flowing at about 1gal/50secs. And you go do something more productive for 4mins. Come back before the end to pick up the can and tilt it to get out all you can with the siphon.
No spring-loaded crap EPA nozzles, no leaking flex nozzles. No standing around holding a heavy weight.
There's lots of copies / versions out there. It's just a swaged piece of copper pipe, with a light spring and marble trapped inside of it. It's like a reverse-snorkel. When you want to start the siphon, jerking the hose up and down in the jug makes the marble block the flow out of the hose so each stroke plunges more fuel into the line until enough is trapped to start the siphon action. SimpleSiphon, SuperSiphon, Super EZ Siphon