I'm not sure if separating the stocks of fuel, art ammo and basics in the base is a good idea. Unless you have enough information to do it in historical quantities.
At the moment the supply is just an arbitrary quantity handled in tonnage, because I assume the historical figures for this at corps and division level are easier to find?
This works quite well for the player, because its like being able to request your own stocking levels, in terms of specific kinds of ordinance (ammo > arty ammo).
For example:
If you suspect that most of your fighting will be done by armour, over open ground against armour, you would tell your quartermaster to ensure a high percentage of your allowed supply tonnage is anti tank rounds (ammo).
If you are fighting infantry in close terrain you would order a high percentage of artillery rounds (arty ammo).
By having the supply converted from the 'push' to whatever ordinance is required when requested by the 'pulling' unit, it allows a certain amount of freedom for the player to simulate that they would have ordered more of a specific quantity of ammunition, because they knew that is what they would need most off.
With regards to fuel, if you are in the desert and have to drive many miles, like in Beda Fomm scenario, you might decide that fuel is more important than artillery ammunition, so order the bases to carry more tonnage of fuel, in place of artillery rounds.
Things might start to run into problems in situations where fuel was historically limited, and take the decision for the percentage of the base stocking levels out of the players hands. You get what you get and if its not enough...to bad.
I imagine basics would be the easier of the three main items to judge quantities for. Just enough to keep them alive will do
This is going to add quite a lot to the complexity to the supply. I'm not sure its worth the work to implement it, especially if your not going to include sliders to allow the player to choose what percentage of the different kinds of supply they want, or be able to find the historic stocking levels of the different quantities.
It would certainly add more depth and complexity (which to me is always a good thing), but I wouldn't put it high on my list of requested features.