I recently got to talk to an engineer regarding plastic tanks and EPA small engine regs. The EPA seems fairly content regarding the efforts to lower emissions on small 2 strokes. Some have cat exhaust, Husky has come out with a stratified scavenging saw, Stihl has out strat scavenge on a cut off machine now, and most likely will use it on other engines as well, as it is much more efficient than a cat muffler. So the next thing is the tank. A blow molded tank will allow out-gassing of fuel vapors into the atmosphere, which is polluting. So more and more the tanks are now 2 piece plastic tanks using nylon and other polymers that will not out-gass. These kinds of plastics can't be blowmolded like polyethylene can, so that is why they are ultrasonically welded. According to this engineer, anyway, and it makes sense to me. So I guess that any one piece blow molded gas can would be more likely to let the more volatile molecules of the gas "leak" out, over time, so the fuel would go stale. A metal can is less likely to allow that if sealed tight. I think that any air, if humid, caught in a metal fuel can would be more likely to let the water settle out and cause rusting in the bottom of the can, so I guess a metal can should be kept full if the fuel will be stored for very long. I try not to keep any for more than a couple of months. If it get's that old I just dump it in the truck. I agree with eyolf that plastic containers vary widely in thickness and quality, but they are cheap, so that's why the sell. It is hard to find a metal can for sale anywhere.