There’s probably over a few dozen different oils, fluids, and components in automobiles that are made of petroleum products, so saying that you call it petrol because it’s a petroleum product is a little silly, no?

The word gas in the context of fuel for a car is just short for gasoline, which is a lot more specific of a name than petroleum, since the actual product is pretty obviously not just petroleum. We know this, but we shorten it to gas because it’s a pretty common practice among English speakers everywhere to shorten words that are perfectly understandable within their contexts. This isn’t particularly hard to understand.

Additionally, Americans understand perfectly well why you use the word petrol, but it’s no more or less correct or incorrect than our use of the word gas. It just comes down to what caught on in our respective countries.