When a moving oil tanker suddenly stops the oil inside the tanker Continues to be in the state of motion due to the inertia of motion.
As a result, it splashes in the forward direction. Similarly, when a stationary tanker suddenly sets into motion, the oil inside the tanker splashes in the backward direction.
If no space is left at the top of the oil, it will overflow. Therefore, to prevent any overflow of the oil due to sudden start or stop of the truck, some space is left at the top while filling the tanker.
