Oil is bare wood.. so once a month or more if needed you'll need to oil the outside to keep moisture from getting into the wood and warping it.
Poly is a coating on the wood that protects it from moisture.. will be glossy and kinda won't feel like wood in the hand. May wear off in spots after years. Can be sanded off if wanted. Though you can clean it with a dang cloth. I use a wood soap. Prob can on either one.. I have no probs
Either one you will need to use mineral oil and a brush to apply on the inside once a month or once every 2 months. Brush everything inside and wait a few mins and then wipe it out.. use a q-tip to get all thr hard to reach areas.
Not needed as often as your not touching the insides like the outside of the wood. I just get it done at the same time when I do the
mods once a month.. usually in the beginning of the month.