Triggering the fuel cost on different values for Combat and Non Combat Move has the big issue that it is charged upon loading, not upon unloading, sadly (Veqryn did this because there were issues with charging it upon unloading, and he said it would have been too much work to put it to work that way, thus he went for charging it on loading, while initially it was charged upon unloading, but was bugged). For example, if you say that Armour costs 1 PUs fuel during Non Combat Move and 2 PUs fuel during Combat Move, which would be an interesting dynamic, then you would pay 2 PUs if you load and unload it during Combat Move, but only 1 PUs if you loaded it during the previous Non Combat Move phase, which makes no sense.
Also I believe it is very weird for non-Americans to see the liquid motor fuel (assuming you are meaning petrol, not kerosene) called gas. I would suggest to at least call it gasoline, or most non-Americans I suppose will think you are talking about some natural gas, like methane.
Just a warning, since I think most not-Americans won't think about something liquid when something is called "Gas".
But maybe it's just me.
The idea is just to let someone develop a tech to spend less on fuel, and I have the techs activating before noncombat move. It might get moved to the end of turn phase, that's not really the important part. I'll consider renaming gas as well
Do you know if its possible to set triggers to change properties like this? The format I have does nothing (as far as I can tell the engine just ignores that line of code). I could have sworn there was a way to change unit properties that needed at least 2 variables, but I can't find how to format it for the XML.
I'm almost sure switching fuel on/off works fine; tho purists would say it's a hack.
As long as you are making a game with no air and no naval there are no other issues either.
The only absurdity is the load / unload I detailed.
With naval, there is the nonsense that a ship staying in the middle of the ocean forever will have no fuel costs forever, which makes the TripleA absurdity of not having to have to go back to port worse off.
With air, there is the nonsense that you use fuel on the fly, which means ridiculous to calculate for what you need in noncombat and maybe you will finish the fuel on the fly and crash, which is silly. Also the engine does not check for all aircrafts having the fuel needed to safely land, which means kamikaze fuel is allowed.
Honestly, at its current state, I think fuel should be used only for land units, never for sea or air units, unless getting very creative with triggers etc. or if it is enough marginal that you never have to worry to consume it all.