Being an old recipe, it probably means normal vegetable oil. Personally I would change to olive oil just because I prefer it and it may have health benefits.
I'm pretty sure it doesn't mean 10-30 motor oil.
x2.... I also prefer an Olive Oil, it does well for salad dressings, much less 'heavy' than most of the other oils (unless you have grapeseed oil, which is also good)