If you oil a rusty bike chain, wipe off and keep doing it tentatively for a while most if not all rust will come off. I'm not sure why it works, but it does. A very rusty or worn bike chain needs to be replaced but even the worst cases clean up rather well. I have used very basic stuff, generic brand chain oil usually with a type of rust inhibitor. I have used a lot of brands over the years with much the same results. If I use WD40 I get rust within a couple of days in rainy weather, it's just too thin and wears off. Like miriam mentions, it might work on a slightly corroded surface.