Why does this transfer box fluid bottle look squished?

Question

I recently bought a few bottles of unopened transfer box fluid for my LR3.

I have no way of telling how old the fluid is, but it seems to have done something over time to reduce the volume of air inside the bottle, either reacted with or absorbed a gas in the air.

I don't know if transfer box fluid tends to absorb moisture, but would be surprised if there is really that much moisture in air.

So what's causing this behavior, and does it affect the meaningfully affect the efficacy of the fluid?

Answer 1

This is usually because the ambient air pressure when the bottle was filled is less than the current ambient air pressure where you are.

The other possibilty is that a temperature reduction will also have the same effect.

Usually a combination of both together though.

This was a classic experiment in physics at school by boiling water then sealing the container and allowing it to cool...