If you live in the country the minimum is 150 Gallons. You can’t get less than that delivered from any company.
I think the reason deliveries of 25 gallons became common was customers weren't ordering at all. The companies were beginning to panic because they were unable to take new inventory deliveries.
I ordered 100 gallons of home heating oil here in small town Maine the other day.
Please see post #28.
We also have a “heat pump”....electric heat. In the summer, it’s a real great air conditioner.