Barrel aging question

[ohiosam]

I've tried searching and haven't found the answer to this question. I want to try a small amount of barrel aging, I'm a small producer and don't want to use all my syrup on an experiment. Does the barrel have to be full or can it be 1/2 or 2/3rds full? I'm looking at a 5 gallon barrel and would like to try maybe 3-4 gallons syrup. I'm concerned that there could issues with spoiling if there is that much air in the barrel.

[Road's End]

The places that sell the barrels (the good ones anyways) purge the barrels with inert gas like nitrogen to keep bacteria from growing while in storage, so maybe that would work for a partial barrel? I've only ever filled them full so I can't say if it actually works.