"I have oil."
Translation:J'ai de l'huile.
In French you can't literately translate "I have oil" into "J'ai huile" like in English. We need a word between "ai" and "huile".
if it's something you can count, you can use a number "J'ai une tomate", "J'ai 4 patates". But here, you can't really count the oil so you have two choices
"le", "la" or "l' ". So "j'ai l'huile" is grammatically correct. But it's more the translation of "I have the oil". You need more explanation to know which oil you are talking about. If your wife ask you to buy her some great oil from Greece and you bring her this oil, you can say "J'ai l'huile' because she knows which oil you are talking about. But here it's not our case.
-"du", "de la", "de l' ". If you use them, they mean "I have some oil", which is the meaning of our sentence there. We don't know which one, we don't know how much, we just know we have oil.
So if you want something easier, just remember that in French there is no difference between "I have oil" and "I have some oil", both are translated into the same sentence "J'ai de l'huile"