Yeah, infantry requiring oil per turn is weird. You could make an argument that infantry could require a flat -20 (or whatever) units of oil when they're built just like iron, horses, and niter units; but oil per turn seems like overkill. Did WW1 to Vietnam era soldiers use vehicles to get around? Of course they did. But most of them, once deployed, did most everything on foot.
If you really wanted to stretch it, you could argue that while that era infantry didn't use much oil, it did cost a lot of oil to supply them. Personally, I don't buy it and I think oil per turn for infantry should be removed from the game.
Regarding artillery, I usually have two maybe three artillery so it's not that big of a deal. Artillery fill a niche so you usually don't need many of them. The infantry thing is a big deal though. Why should anyone ever build the entire melee line of units and NOT the heavy/light cavalry line? Light cavalry are great in that horsemen are very good and only helicopters have a maintenance per turn. That maintenance is NOT oil but aluminum instead meaning you're not dipping into your tanks and artillery's resources. The only other units that use aluminum are air units and I almost never build them because early air units have such terrible range and I've basically won the game by the time fighters and jets come online.
I like that units require resources and I don't mind the oil per turn either. I just really think that it shouldn't apply to infantry. The scrabble to get more oil is a neat sub-strategy but it too often falls victim to RNG in addition to too many units requiring oil.