His example said a week. If someone gives you their car for a week and you got it on like a half tank, yeah, giving it back full is kind of a thing. A 'thank you', a "least I can do" for the use of a vehicle for 7 days. That's why he also said the rental costs, and how it'd be like 1/14 of what you saved.

It's a gesture of appreciation. Because even if it's not a week, if you loan out your car only to get back into it in the hole on fuel, that's just really shitty. You were helpful and they gave it back with an "eff you" and it will actually cost you money to get back to where you started. Same goes for lawn mowers and weed whackers. This is an example of "adulting."