Is there an existing feature request for this?

• I have searched the existing issues

Describe what you would like added or changed

As for now fluid boilers only work with creosote, would be nice to be able to add different fuels from other mods with a datapack or an option in RR's config, allowing to add fluid tag and the amount of heat they produce.

Is there any update on this ability being added, or if it has been added, which version was this added in? Being restricted to only using creosote when some modpacks have 6+ fuels... Would really like to be able to set the firebox to be able to accept different fuels.

Hopefully this is being worked on and is scheduled to be added rather soon.

Sorry for being late.
So, this feature already exists in minecraft 1.21+, you can add custom fluid or fluid tag via datapack.
You can edit this file: data/railcraft/data_maps/fluid/fluid_heat.json and add for example another fluid:

{
  "values": {
    "#c:creosote": {
      "heat_value_per_bucket": 4800
    },
    "minecraft:lava": {
      "heat_value_per_bucket": 1000
    }
  }
}

Unfortunately 1.20.1 does not have Data Maps and this would require too much work to implement a similar management. Work that is better focused on feature parity with 1.12 and bug fixes.