How to reduce troops oil consumption?

There are several way to reduce fuel (oil) and food consumption for your troops.

Become a trader: Traders get 50%less troop consumption, but there are downsides to this class as well.

Dismiss troops: Let troops get wounded and dismiss them in the hospital to reduce troop numbers.

Use heroes: Development Heroes that can be stationed in the Troop training buildings (fighter, vehicle, shooter training camps) have Skills that reduce food / fuel upkeep. Recruit these heroes and unlock the skill. Orange heroes are Drill Master for fighters, The mechanic for vehicles and Master Marksman for shooters.

However you will find as you progress that unless you keep your troop numbers very low you will always have a higher consumption that production. The game does help you here and give you a threshold that you will not consume resources below. (so you will always have some oil to use). The solution is to keep earning rewards from events that give you stored resource items. These inventory items are not used and will remain in the item store until you need them.