Originally Posted by cyccommute

Penetrating oils are just a low viscosity oils. Nothing corrosive in them. Most rust and corrosion has a lower density than the metal it forms from so the oil penetrates into the threads and lubricates them. This allows the parts to be separated because they can now slide past each other.

Iron oxidation (aka "rust") has a very porous nature so the oil can penetrate into it better than other kinds of oxidation. Rust is also softer than the metal it is formed from so it tends to break when force is put on it. If the iron parts can slide past each other, they can come apart easier.

Aluminum oxidation products, for example, are extremely hard and many even be nonporous. Think anodized aluminum. The aluminum oxide is also much harder than the underlying metal so it doesn't break and slide as easily. Penetration oils often don't work on aluminum parts for this reason.

You could leave the oil in place as it will still lubricate but the oil is so low a viscosity that it will eventually flow away or evaporate. Putting something like grease on the threads after you've gotten them apart will go a lot further towards keeping the parts from welding back together.