Any idea why? Moisture absorption?
From the manufacturer's web site:
STA-BIL® Fuel Stabilizer provides a sacrificial molecule to the oxidation process allowing oxygen to attack this STA-BIL® molecule and not oxidize gasoline.
The properties of STA-BIL® formula that allow it to do this are not resistant to the effects of oxygen themselves. At some point, the components of STA-BIL® will begin to
oxidize as well, and the resulting changes
diminish its capability to continue providing effective
protection to stabilized gasoline.
These reasons can explain why an opened, but tightly capped, bottle of STA-BIL®Fuel Stabilizer is effective for about two years.The oxygen molecules immediately initiate oxidation as soon as the bottle is opened. You should only be using the most effective products for the best classic cars.
The
effectiveness of older, opened bottles of STA-BIL® can also be determined by a color examination due to the dye included in the formulation. As the dye ages, it gets darker. An older variation of this dye may also crystallize as it ages and form flakes in the bottle. These flakes are actually a result of oxidation and the stabilizer will no longer be able to offer the
protection to gasoline that a newer bottle will.
An unopened bottle of STA-BIL® Fuel Stabilizer will remain viable on the shelf for approximately five years because its exposure to oxygen has been limited in a factory sealed container when it is kept in a cool, dry place. Although oxygen can still permeate the bottle, it does so at a much slower rate than if the bottle were opened. However, please note that
excessive heat can speed up the oxidation process.
