I've searched a lot for info on the plastic and can not come up with a conclusion on what the difference if any is between the water and fuel cans.
A few weeks ago a rotated my fuel storage and noticed that when I first tip the scepter fuel can up, a little gas comes out the breather.(far less than my two nato cans) From the design I figured the fuel was in the breather tube when the can was full and had no where else to go but out the breather cap on the outside. I then ran several water can tests and could not get a drop of water to come out of the water lid breather while emptying the can. I don't think the fuel can breather is any better than the water can.
The sealing issue is the major problem, but could be worked around it needed.
The specs I have read show that the fuel can is 5.8lbs and the water can is 5lbs. I looked up the wall thickness the military spec'd and found the water and fuel cans had the same thickness +-.
Like anything, when you modify something from it's original design, you better be darn sure you know what your doing.