Office of Steam Forum for Model & Toy Steam Gas & Hot Air Engines

There is a huge difference actually, in that hot steam easily "floats & strips" regular oils away.

True steam oil contains a small percentage of organic fats that have the ability to "stick" to the hot, wet surfaces to maintain a layer of lubricity in spite of the hostile environment.

However, on a different note, low pressure and temperature steam that is "saturated", which is what our toy steam engines operate on, actually does provide a modicum of lubrication, in that there is constantly a fine "skin" of water coating all the surfaces within the cylinder.
In a real steam engine operating at high temperatures and pressures, there is no such water in existence within the cylinders, as the steam is actually "dry" in that situation, besides which the forces involved in such an engine would far outstrip the meager lubrication ability of water, which can only work as a rather poor lubricant when there are no significant forces involved.

If you are running at just an atmosphere or two of pressure, you are likely fine, but as the pressure goes up and the sophistication of the engine increases, true steam oil does become important. Like the difference between a small Mamod and a large Stuart.

Depending on what you are actually running, you are probably fine using air tool oil, or no oil at all, but in the long run it would certainly be better to use a true steam oil, even for our little toy and model engines.

Hope that helps.