Now oil icons of each tier use the same icon, just like bombs and potions. Why? Because...
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Author's instructions
You can do with this whatever you want. Re-use it, change it, whatever you want. I don't care.
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... before any monster fight I would be looking at the bestiary to see what kind of oil I had to use. However the bestiary always shows the icon of the normal oil, regardless of the oils in your inventory. So then I always ended up searching for that icon from the bestiary in my inventory, but couldn't find it because I had already upgraded to enhanced or superior oils. Now they all look the same and are easier to find/remember (to me at least).
How to install:
Download and unzip the main file
Move the folder modUnifiedOilIcons to <game_path>\mods
How to uninstall:
Remove the folder modUnifiedOilIcons from <game_path>\mods