As France you get Garde Imperials which are as good as Infantry on your own continent and only cost nitre. There is zero point in building leg infantry when you could build tanks for the same oil cost. Mech Infantry are nearly as good as Modern Armor so it is worth upgrading musket men to them but you would build MA before MI. The only reason to have infantry is they get +10 against all the AT troops the AI loves and you should love if you don't have much oil. AT troops and machine guns make excellent garrisons. All ships and land units that consume oil should be made into armies and armadas as the oil cost is the same as a basic unit. The AI organises into armies too so basic units are not that useful in late game. A city capture team of 2 tank/MA armies and 1 artillery/rocket artillery army with balloon/drone is effective and can be maintained on 3 oil per turn or 1 well.