'Oil' and 'petroleum' may be used sometimes with the same meaning, when we're talking about the raw material. Oil rigs. Drilling for oil. Oil spills. The worldwide price of oil. Oil companies. Petroleum is the more accurate term, however. Oil is also a more general term, for mineral oils used as lubrication, or plant-based oils used in cosmetics and foodstuffs. Petrol is the UK term for the fuel (made from petroleum) used in motor vehicles. It's the equivalent of the US term gas or gasoline.

You're right, it depends on the situation. When we are talking about filling up your car, we use the words 'petrol', 'petrol prices' and 'petrol station'. I might be wrong (and someone please comment if you think there are more uses!) but I am pretty sure this is the only time we use the word 'petrol'. The rest of the time we refer to it as 'oil' in general - for example 'oil company', 'oil spillage', and so on. I hope this helps a little!

"Petrol" is British English. In the USA, the word "gasoline" is used. Petroleum is the word for oil.

Petrol = gasoline. Petroleum = Oil Petrol is a fraction distilled from oil, so petrol and oil are not inter-changeable.

As Randy said, we don't used "petrol" in the US. It's "gas" or "gasoline" (or "diesel" or "diesel fuel", if applicable.) "Petroleum" is the product pumped from beneath the earth that gasoline is made from. "Oil" is a more generic term that includes not just petroleum but products made from other sources, such as "vegetable oil" or "walnut oil".