You can right click the game in your library, a menu pops up, here's what to do: Manage > Browse local files, then it takes you to the files, then go: Flyout_Data > StreamingAssets > Fuels, you can then drop any fuel files you find on discord, there.

If you are looking to replicate fuel, research it and fill variables accordingly, if not, make your own, here's how to do both (this is as far as i know)

(you can copy a file and change things to get what you want)

Take a look at the screenshot of what you will see in files,

https://steamuserimages-a.akamaihd.net/ugc/2261431663692647309/5B5C6C6EADAC75F0D432A128A7679596ACEC0FF3/?imw=256&&ima=fit&impolicy=Letterbox&imcolor=%23000000&letterbox=false

here is what they mean:

id: fuel identification number (cannot be the same as an already existing fuel.)

name: In game name

density: Fuel density, volume/weight in g/cm³

specific_energy: {joules/kg from burning}

heat_capacity: This is the specific heat of the fuel in Joules per Kelvin per kg. It's the amount of heat in Joules needed to raise the temperature of 1 kg of the fuel by 1 K.
It's basically a measure of how much heat it "steals" from the combustion energy to raise its own temperature. Thanks to Sundae for clearing this up.

stoichiometric_ratio: stoichiometric ratio is the ratio of air and fuel needed for efficient combustion, IE the ideal fuel to air ratio for gasoline it is 14.7 parts air 1 part oxygen, and its different for other fuels, like for nitromethane its like 8.3:1 (thanks to frenchfry for the comment)

auto_ignition:Auto ignition temperature. In kelvin

freezing_point: Freezing temp in kelvin


Thanks for reading, if you have further questions or inquiries my discord is "mrthermonuclear")

https://steamuserimages-a.akamaihd.net/ugc/2261431663685923339/08E326D493E0418D3D7265BDC4F65DDABB1C2FF5/?imw=256&&ima=fit&impolicy=Letterbox&imcolor=%23000000&letterbox=false

hope to see your custom fuels in the flyout discord!