Storage facilities for crude oil and petroleum products require a minimum volume of product in their tanks and pipe infrastructure to remain operational, which can vary between different storage facilities or tank systems. If storage levels fall below their minimum volume, pump suction can become ineffective and storage facilities cannot function. These minimum volumes are sometimes called tank bottoms.
from the linked article
our comrade mentioning sediment is correct as I understand it. Or at least that's why i was told to not run a petrol car to an empty tank
A world where people read Da Rulz is still possible