There's a metaphor in this I think

Our mills require a constant oil feed to the spindle for maximum efficiency, like most machines. The oil baskets are pretty small and at an awkward place to fill with a pump, so we use a canister to move oil from the drum into the basket.

The canisters are kind of an awkward shape, it's really hard to fully pour them empty even with a funnel hose at the mouth, so even while they're pretty heavy when full, it's better to fill them over halfway so you don't have to awkwardly tilt them at a 90+ degree angle to get the oil into the basket.