There are many different resin formulations of plastic (that's what the number in the middle of the recycle symbol on the bottom of the container stands for, for example) and not all resin formulations will tolerate having used motor oil stored in them without breaking down and eventually leaking.
I'd stick to either storing the used oil in used/empty motor oil bottles (you know they won't leak since they are used to store motor oil already) or in something water-tight that's made of metal (like a barrel or a designated used oil container).
Yes, but if you ever want to store water again in it, you will be hard pressed to get it clean.
NO, since used oil is considered "toxic waste".
P.S., I meant to say "not legally". You can store it anywhere you LIKE, as long as you're not caught.
In a word,yes. There may be EPA regulations on where and how to do it, you may or may not have to comply with.
