Gasoline and petroleum are different products as pointed out in the sibling comment.

You may be thinking of petrol, which is what gasoline is often called outside the US.

The same reason why Russian gas is still getting transported through Ukraine’s territory and why Ukraine gets transfer fees from Russia for it.

Oil is pretty fungible so it is easy to bypass sanctions with oil as you describe. Saw an article about it recently. There is a difference between petroleum (oil) and gasoline (refined product)

Oil chemistry can determine country of origin and sanction enforcement is possible at port.