Oooh, oceanic language barrier alert!
Petroleum is a general term for all petroleum products in liquid form. Legally, anyway. Gasoline and No. 2 Fuel Oil are both "petroleum." An even broader term, "Petroleum products" range from gasoline to the plastic monitor casing you're looking at right now.
Petrol means the same thing as fuel and as gas in their vernacular uses in various countries. In Europe, you'll need a translator if you want to buy "gas" from someone. Here in the U.S. using the term "petrol" would let everyone know you're British, or wish you were.
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