Newps wrote:

Those are all vented, although depending on the year the newer cars vent
into that coffee can looking container under the hood. A small portable
tank is only vented when you open the vent to pour out the gas. Close that
vent and it is airtight.

I was referring to the small tank on my lawnmower, which has a
vent in the gas cap. It will "go bad" in the 8 months that it
sits unused. If I drain it completly, or pour a few oz of
Stabil in it, then it will run ok the next spring...

MikeM