TileLayer

local fuelSlot = 1

turtle.select(2)  --start on first slot
local stackCount = turtle.getItemCount()

function updateStackCount()
  stackCount = turtle.getItemCount(turtle.getSelectedSlot())
end

function swapSelected()
  updateStackCount()
  if stackCount==0 then
    currSlot = turtle.getSelectedSlot()
    currSlot = currSlot + 1
    turtle.select(currSlot)
  end
end

function checkFuel()
  if turtle.getFuelLevel() <= 1 then
    prevSelected = turtle.getSelectedSlot()
    turtle.select(fuelSlot) --fuel goes in this slot
    turtle.refuel(1)
    turtle.select(prevSelected) --use any slot here, I default to 1, depending on the program.
  end
end

while turtle.back() do
  checkFuel()
  swapSelected()
  turtle.place()
end