Hey - I know this hasn't been updated in months. I also use the betterloot tool : https://blmc.artemdanilkin.ru/  I'd like to add diesel to the fuel barrels. However I don't see it updated to the list yet. :( 

You just ad it in the loottable (/oxide/data/BetterLoot/LootTables.json)
So it goes from this:

    "assets/bundled/prefabs/radtown/oil_barrel.prefab": {
      "Enabled": true,
      "ItemList": {
        "crude.oil": {
          "Max": 35,
          "Min": 5
        },
          "lowgradefuel": {
          "Max": 50,
          "Min": 5
        }
      },
      "ItemsMax": 3,
      "ItemsMin": 1,
      "MaxBPs": 1,
      "Scrap": 0
    },
​

to this:

    "assets/bundled/prefabs/radtown/oil_barrel.prefab": {
      "Enabled": true,
      "ItemList": {
        "crude.oil": {
          "Max": 35,
          "Min": 5
        },
        "diesel_barrel": {
          "Max": 3,
          "Min": 1
        },
        "lowgradefuel": {
          "Max": 50,
          "Min": 5
        }
      },
      "ItemsMax": 3,
      "ItemsMin": 1,
      "MaxBPs": 1,
      "Scrap": 0
    },
​

The item ( "diesel_barrel": "Diesel Fuel" ) should already be in your nameslist (/oxide/data/BetterLoot/NamesList.json) so it should work as soon as you reload the plugin.