tim is usually a mixture of oil + metal or some form of carbon dust mixed into a gloop.
It can dry out... the oil is not everlasting.
However if sealed, it will take a very long time to dry out.
The great thing about tim is that once u apply it and put the cap back on, the left over tim will seal the cap air tight.
This helps it to not dry out.
This is why its best just to squeeze some out and check the viscosity of the TIM.
If its like toothpaste, the oil has not dried out and its still good.
If you need to squeeze the thing hard enough to give u a hernia, to get something out, well, without doubt, its done and should be tossed.