What happens when lava and oil mix?

Answer 1

Lava and oil don't mix, any more than lava and seawater mix.

While the lava surface will be cooled slightly by the oil, there is a lot of heat in lava and the oil surface will be heated rapidly, leading to vapourisation and probably burning, as most types of oil are flammable.

The outcome will depend on the volumes of the oil and lava, but as there's likely to be a lot more lava than oil, the oil will vapourise and probably burn.

Water is much more effective at stopping lava. Water is available in large quantities, it cools lava down and makes it set, while vapourising as steam without pollution.

Because of the amount of heat in a lava flow, it would take vast quantities of water sprayed across the face of the lava to stop it, and the resulting wall would be increasingly fragile as it grew higher, as only the surface of the lava would be cooled, not the pool of molten lava behind it.