I've only played a few games in the past couple weeks, waiting on the next update before I go to town again trying to pick apart the difficulty settings vs AI opponents. But I do have some more thoughts on fuel.
Right now the starting fuel reserves for both sides are nearly 3 times the recurring fuel income per turn. So for example, Russia starts with 80 fuel +32 per turn. Japan starts with 90 fuel +38 per turn, ANZAC starts with 20 fuel +6 per turn etc. Honestly I think the ratio is weighted way too heavily towards the starting reserve, and not enough on + per turn recurring fuel. The starting fuel reserve is dramatically larger than the fuel income that can be achieved from the barrels on the actual map, but the player (esp. a first time player) is unlikely to notice just how stark the disparity is until the reserve is exhausted which takes sometimes 3 or 4 rounds of play. Germany for instance starts with 80 fuel, but in order to ever see a total like that on + fuel per turn they'd basically have to conquer most of the globe, so once its spent the starting reserve is essentially unrecoverable. Since the game is based on movement at bedrock this makes that starting fuel reserve (how you spend/save it initially) the single most important thing going on in the whole game.
The issue I see is that the new player just doesn't have enough time to settle in and get used to how fuel works before it totally dominates the gameplay and they have to learn the hard way. Basically its a situation where the first and second round can accommodate casually moving just about everything on the board, but then you see a really hard crash in the third and fourth round after which point the fuel per turn can't cover a substantial number of the units in play. From round 5 on most of the larger powers can only move like half their stuff in a given round and some of the smaller nations barely have enough to move even a couple steel units before running dry. I think the fuel movement works against the entertainment factor here mainly because from the player pov it occurs really suddenly, more as a crash than something gradual that you can figure into your planning, and so potentially more frustrating than fun, since its harder to see coming until you're all mired in the problem hehe. But again, I think its more because the starting fuel reserve is so large relative the recurring fuel per turn, and not the fuel mechanic per se that leads to the disconnect.
I'd consider coming way down on the starting reserve, so that fuel is a factor right from the outset and a critical consideration in the opening round gameplay, then bringing up the +fuel per turn by a significant amount, like 25% at least, maybe 50% more to cover the disparity from having a smaller starting reserve. That way the learning curve isn't quite so steep, and the player has some time to get used to the fuel/movement restrictions before seeing a hard crash off the cliff. In other words, having the player immediately confronted with the fuel thing right from G1, instead of pushing it out into the midgame, so the player can see what's happening right from the outset and say to themselves 'oh wait what's going on here? I guess I need fuel to move? I better figure this out'. Then start adapting to the new mechanic right away.
For that to work at its best, ideally each nation should have to make at least one important fuel/movement decision in their opener, so that each player is immediately introduced to the core concept that will be governing the gameplay for the rest of the match. So an example might be on G1, the player discovers that they don't have quite enough fuel to move every single starting unit and must choose between say moving the bomber, or moving a couple mech or moving a few ships. Same deal with J1, where maybe Japan can move 'almost' everything but not quite all of it, so the whole fuel=movement thing comes into play immediately.
Right now the way its set up, all players depend on the much larger starting reserves to see them into the midgame. Even planning way ahead and only moving what is absolutely necessary from the get go, the ultimate limit usually comes from the size of the starting reserve and how quickly you do or don't burn through it. But if the +fuel per turn was increased it would feel a lot less do or die, and players would have an easier time recovering from a poor decision in early rounds (moving a large fleet too early, buying more armor than you can afford to move etc.) Basically putting in more of a buffer over time, rather than front loading it with larger starting reserves.
The optimal target range would be enough +fuel per turn so that new units can still enter play and be functional, higher than the current, but with the movement spread per round being narrower. So rather than a choice between moving like just half the existing steel unit force, it'd be more like just a few units in a given turn that would fall out of range. Same deal on the purchasing side, where you'd be acquainted with the fuel dynamic sooner (right from the opener), so you'd have a better feel for how a new purchase plays into the fuel requirements.
I think the current recurring + fuel per turn is just too low to really get a feel for things before it suddenly becomes the whole game, so basically trying to stage it in more gradually if that makes sense. I think the easiest way is to come down on the reserve based on the movement needs of the openers, and then put the +fuel per turn at right around that amount (little more or a little less) depending on which nation it is, whether they are meant to expand into oil territories right away, or giving them up to the other side after the openers.
In the save below you have Germany doing a max move, and ends up using just about...
20 fuel on mobile ground (maybe about 25 if all the armor/mech move 2 tiles.)
10 fuel on aircraft (closer to 20 committed to land.)
25 fuel on ships (if you move all the naval units.)
So ballpark, something around 70 fuel to move all their starting units?
In that case a range of maybe 55-65 starting fuel would probably force a fuel/movement decision in the opener. So either leaving some ships in position, some armor at move 1 instead of move 2 etc. Things like that.
Assuming the attrition rate for steel units is pretty low in the opening rounds (most will survive the opener to keep moving each turn) then that probably also gives a decent ballpark from how much G should be seeing in +fuel per turn. Buying new steel units beyond that means that some existing units would either have to stay in position or die in combat to make room, or else the new unit stays in position until enough fuel is conquered or saved up to get them moving out together.
I know having large starting fuel reserves might feel thematically cooler (since its easier to rationalize as pre-war stockpiling of fuel), but for the gameplay and ease of use I think it'd be much better to have the player learning to manage fuel as they go, incrementally, with more guideposts out of the very first turn. I think that would be an easier sell for the newb, instead of the current situation where they kind of have to be already familiar with the mechanic, and able to prognosticate about fuel consumption far in advance in order to play well (like an expert fuel barron), which is what the practical effect of having such a large starting reserve relative to the recurring totals per turn amounts to.