oilyoudaoicibaDictYouDict
oil: [12] Around the Mediterranean in ancient times the only sort of oil encountered was that produced by pressing olives, and so ‘oil’ was named after the olive. The Greek word for ‘olive’ was elaíā, and from it was derived elaíon ‘olive oil’. This passed into Latin as oleum, and reached English via Old French oile. By now it had begun to be applied to similar substances pressed from nuts, seeds, etc, but its specific modern use for the mineral oil ‘petroleum’ is a much more recent, essentially 19th-century development.
=> olive
petrolyoudaoicibaDictYouDict
petrol: [16] Petrol originally meant ‘mineral oil, extracted from the ground’ (what we would now call petroleum or, more loosely, simply oil); not until the end of the 19th century was it applied to the ‘fuel refined from this’. The word was borrowed from French pétrole, which in turn came from Latin petroleum (itself taken over directly into English in the 16th century).

This means etymologically ‘rock-oil’. It was formed from petra ‘rock’ and oleum ‘oil’. Other English words that go back to Latin petra or its Greek source pétrā include parsley, petrify [16], saltpetre [16] (so called because it forms a crust like salt on rocks), and the name Peter (a reference to Jesus calling the apostle Simon the ‘rock on which he would build his church’ – hence ‘Simon Peter’).
=> parsley, petrify, saltpetre