Yep, there's a lot of reasons why, at work, we give dry chemicals a 2-year expiry and solvated chemicals ~30 days (even in hydrophobic solvents like hexane). Even crimped vials are not airtight. I have proven this by stability testing over time using Karl Fischer titration.
In my opinion, the best way to store gear long-term is to divide the bulk amount into amounts that will be brewed in 1 batch. Vacuum-seal these amounts individually and wrap in foil to protect from light. Store in the freezer. The reason for this is because when something is thawed, condensation forms. This way, only the amount of drug that will be immediately used is exposed to condensation.
As you,
@SidneyPascal and
@Laxbro88 have said, storing in oil is no bueno, since the oil will go rancid before the drug degrades to any noticeable extent.