Best thing probably is to exclude air from the storage bottles, squeeze it out before replacing the cap, and ensure the bottle is airtight. My ID-11, made up 1:0 in August 2008 has been stored in 500ml plastic (PET I think) pop bottles, and it's good to go. Oxygen absorbed from the atmosphere will slowly oxidise the developer. I don't know what effect 100% butane would have. If the dev significantly darkens, it's probably stuffed. It works for me, but YMVV.

Cheers,
kevs