TL;DR
Change the fuel inventory in vehicles, furnaces, boilers, assemblers and inserters to use the same interface and features as cargo wagons.What/Why ?
Cargo wagons have features that would be useful for managing fuel in other entities:1) The inventory size can be limited so inserter only fill a limited number of slots.
This would be useful with locomotives when using e.g. nuclear fuel. Except for extreme cases a single slot for nuclear fuel is plenty and saves stuffing each locomotive with fuel that is never used. Users can still dump excess fuel into the blocked slots manually though (and it will be used up first). Other fuel sources might be ok with less slots than the maximum too.
2) Slots can be reserved to accept only specific items.
Some mods have recipes that take a burnable item as input while also requiring fuel to function. One can fill the fuel slot with the desired fuel type and the fuel type will get used. But if fuel ever runs low the furnace/assembler suddenly switches to burn the ingreedient as fuel and will never recover without manual intervention.
By setting the fuel slot(s) to a specific fuel item this could be avoided.
For vehicles this would also allow to specify a mix of fuel types. For example the last (first if the locomotive keeps using the first slot first) slot could be reserved for wood to burn up any excess wood you dump at the refueling stop first before reverting to the locomotives normal fuel type.
3) The last slot is used up first.
Currently fuel inventories seem to use up the first slot first and when empty all slots shift one to the front. This means loading a better fuel type into a locomotive will take a long while before it takes effect, all the old fuel has to be used up first. In case of nuclear fuel this makes using a mixture of fuels to reduce the number of nuclear fuel per locomotive impossible. Because of the stack size of 1 for nuclear fuel the dummy fuel used to block the other slots keeps shifting to the front.
When nuclear fuel is rare one also can't easily fill the locomotive with a backup fuel like coal. Again due to the stack size of 1 a slot either has nuclear fuel in it and won't take a second or is empty and will accept coal. One has to resort to multiple fuel stops, the first loading nuclear fuel and the second loading coal to have a backup fuel source. Or one has to resort to circuit logic AND overriding the stack size to insert one fuel at a time. Something that makes loading a stack of wood rather slow.
Using the first fuel slot and shifting slots when the first is empty seems inconsistent with other inventories. It would be kind of irrelevant if we had 1 + 2. But since we don't this makes it impossible to get close to the features of 1+2.
