I just added my diesel heater in HA, add wanted to add the correct device_class, but there is none, the only one with eg Liter is Water and i have search everywhere and also asked on discord…
So please add other kinds of Fuel, Petrol and Diesel… And in the energy page i had to make my diesel heater as water and the diesel
Your request is reasonable and valid.
Until someone is so motivated to work on it, you can use ‘Manual customization’ to get your units looking OK.
Customizing entities - Home Assistant.
If you think it’s a good idea, you should vote for it. I did…
Suggested Reading:
Feature Request Guidelines 
- Feature Requests - Home Assistant Community.
Pretty strange that device_class “fuel” is still not added to HA. In the meantime, I declared device_class as gas in order to display it in energy panel.