DOT Brake Fluid
The shelf life of glycol-based DOT brake fluid is very poor due to its hygroscopic properties, which is why every bottle should come with an air tight foil liner covering the opening. As soon as this air tight seal is broken it will start absorbing moisture from the environment and the boiling point will begin to drop.
Castrol recommends that DOT fluid in previously opened containers should be discarded after 12 months. In our opinion it would be unwise to use DOT fluid from a bottle which has been open for more than a few months, unless you have no other alternative.
To reduce large quantities of wasted brake fluid, use DOT fluid from smaller 100ml bottles. 100ml is enough to bleed a set of brakes (front and rear) 2-3 times.
Since Mineral Oil is hydrophobic and does not absorb water from the environment, one of the great advantages it has over DOT fluid is that once opened it can be stored indefinitely.