Initially, your only light source in Call of Cthulhu is the main character's cigarette lighter. However, at the end of the third chapter, the hero finally receives a very useful object - Storm Lamp. It gives several times stronger light than a cigarette lighter, but there is one problem with it - it has a limited operating time, because after a few minutes its oil supply runs out. Where to find more oil to fill it?
This operation is in fact quite simple - you'll have to find a special oil tank. From the aforementioned moment in chapter 3, the tanks will be scattered in key locations in the game. The first one can be found during the third chapter - in the place where you meet Officer Bradley after the pursuit of the mysterious cultist.
To refill the lamp, you need to approach the tank (shown in the picture above) and simply press the interaction key. It doesn't matter whether you're currently using the lamp or the lighter - the lamp will be refilled anyway. This allows you to use the lamp almost endlessly - if you know where the tanks are - and to illuminate the surrounding area properly.
Note - remember, however, that at some points in the game you will not have easy access to oil. So if you don't need a powerful light source, use the lighter, leaving the lamp for emergency situations.