I agree with previous comments generally. Need to go is to have reason to go. Gonna go means you are going to go (even if you want to just look around without any real purpose) Also note "gonna" is generally regarded as an Americanism and is not used much if at all in the UK and may even be frowned upon as many Americanisms are! "Have a nice day" (being another good example)

So custom demanded means that the activity is traditional and they are following tradition. The tradition in this case has been personified for effect, so that rather than "they followed it," "it commanded them" to do something.

I agree with the second answer. I would use neither. Preserve is too formal, save in that sense is archaic (Save as in "rescue" is more popular) Keep or put would be much more common. I am assuming the container is being used to stop sparks flying out of the firevabd igniting the fuel that is meant for storage.