Engine oil has additives and there is the problem, over time the additives break down. This is evident by dark brown or black streaks of sludge forming in the bottom of the container. Additives in oil start to breakdown after about two to three years of storage, sooner if the additives are put to work in the engine hence the ‘or 12 months’ recommended engine oil service interval which is less to do with the oil and more to do with the additives turning to sludge which the oil filter and oil pump cannot deal with. Just empty one container of oil before opening a fresh container and, assuming you keep on top of your oil changes, you should never have any engine oil more than one year old.
Stored brake fluid now there’s a proper can or worms! As above brake fluid is hygroscopic so if allowed it will absorb moisture from the air. DOT4 brake fluid should remain in specification with as much as 2% moisture content. You can test the moisture content with a cheap as chips brake fluid moisture content pen, they’re not scientifically accurate but will give you some idea of what’s going on in your brake fluid. That said do as I say and dispose of any left over brake fluid together with the old brake fluid at you local recycle centre or, do as I do but at your own risk!
I store left over brake fluid in a clean, sealed jam jar of a suitable size so that there is a minimum air gap between the fluid and lid, there is very little difference between the way I store the fluid in a jar and the way the fluid is stored in the brake fluid reservoir. I use the stored fluid to replace the fluid I bleed off when inspecting the brake and clutch fluid at the 4,000 mile service intervals or when replacing brake pads. When next I come to replace the fluid in the brake and clutch system I also replace the fluid stored in the jam jar.