Just this "<font size="2" face="Times,Times New Roman">Never allow fuel to stand open, to be mixed, or to be left in a plastic container for longer than 2 hours, as it can ruin the fuel. Keep fuel in a tightly closed metal can.</font>"

It relates it to 'standing in the open,' and my guess is that the ether permeates through most plastics, that and since most plastic containers are semi transparent, sunlight can get to it very quickly.