As an answer to the unopened sealed bottle thing, kinda depends on how thick, and what polymer plastic was used in the container, and seal... most are made from HDPE High Density Polyethylene, and can cope with quite a bit of resistance to air intrusion, basically because it was completely filled, with very little airspace, and usually stored at a constant temperature... but once opened, and half is used, the remaining "void" in that bottle contains air, and very likely moisture.
a lot of my "opinions" on air and moisture come from my engineering experience when I was responsible for designing "packaging" material for use in "flat pack" Lithium Ion battery production. The chemicals, and composites used to create prismatic cells, are extremely hygroscopic, and as of such, the cathode/anode lamination's once the plasticisers are extracted, are dried, and all "cell assembly" was done in an inert atmosphere (i.e. a helium filled gloved dry box) which was monitored, and ingress and egress chambers were evacuated during loading process. Moisture was virtually non existent, and oxygen also.
The materials I developed for the "soft" package of the cells as designed consisted of multiple layers of non permeable aluminum foil, and proprietary plastic films, which were conducive to laminating with the foil layers, and resultant "heat sealing", I was a great challenge, and I expended a lot of time to perfect the process' (yeah, I have patents on some stuff... but I'm not rich from it... dangit..)
I know it's a bit anal with regards to brake fluid, but I did become very aware of gas and moisture permeability of simple common plastics.
so, In the end, Like I noted, I flush with the slightly aged fluids, and toss them, and my final fill is always fresh stuff... it's cheap, why not.