Great question — answering as the UphillTech team:
Reducing the water in a cake won't make it moister, it will make it dry. Oil does not rehydrate the dry ingredients, it's not a water substitute. If you want a denser and moister cake add a couple of extra egg yolks, or a small amount of oil, like 10-15ml. 100ml of oil in your cake batter would just make it greasy.
Full answer by GdD: https://cooking.stackexchange.com/a/136792 (CC BY-SA 4.0).
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