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All engines with moving parts, steam powered or otherwise, require some form of lubrication. Oil is usually used for this purpose, and these oils are usually petroleum based, although vegetable and animal based oils are sometimes used for special applications. (whale sperm oil, castor oil, tallow, olive oil all have lubrication properties and are used).
There is also some confusion over the terminology used for "Steam Oil". Terms used include "steam oil", "cylinder oil", "'ordinary' steam oil", "'compounded' steam oil", and probably other variations. What is the difference between these?
But first a little bit of history:-
It was quickly discovered early in the steam age that steam engines quickly required some form of lubrication. It was quickly discovered that beef tallow worked quite well, but tallow has the undesirable characteristic that it was full of free fatty acids that form corrosive acids when decomposed in the presence of steam. These acids attacked the metal parts of steam engines. And animal-based oils, by themselves, are not overly stable in air, or in higher temperatures, and tend to become rancid.
Tallow is produced as a by-product of animal carcass rendering, and today the refining processes have improved markedly since the early days, and most of the corrosive free fatty acids in today's tallow products have been almost entirely eliminated.
Mineral-based (petroleum) oils are desirable because they offer greater stability at higher temperatures, and less viscosity change. But they are less than ideal for steam use directly because, unlike tallow, mineral oils do not mix with water, and the wet steam (and inevitable water) washes it off the sliding surfaces that now have no lubricating oil film.
Experimentation over the years proved if a small amount of tallow was added to a mineral oil, the result is a "compounded" oil that will "wet", or spread over all the rubbing surfaces and resist the washing effect of water droplets in the steam. This is analogous to washing your hands with soap. The soapy emulsion helps "wet" your hands, cut through the natural oils and lift impurities from the skin. Similarly, the addition of tallow allows the oil to 'stick' to the surface of the sliding parts and resist being washed off. It was quickly found that only a small amount of tallow was needed to form adequate emulsions. The result is a "compounded" oil that will "wet", or spread over all the rubbing surfaces and resist the washing effect of water droplets in the steam.
Compounding (which is simply a term used to describe mixing several components) produces a family of specialty oils to lubricate the moving parts within the valve chests and cylinders of reciprocating steam engines. Steam oils today typically contain 5%~10% refined tallow (5%~10% compounding). In practice the petroleum producers include several compounds in steam oil to help stabilise viscosity and lubrication properties, hence the name compounded steam oil.
So to get back to the question of what is the difference between these oils, so called "steam oil" is always compounded oil. It should also be noted that the colour of steam oil is determined by the origin of the base oil, and has no bearing on its lubrication properties, and so it is incorrect to assess steam oil by its colour.
So called 'ordinary oil' probably doesn't exist, as most of the readily available automotive or machine oils are also compounded, but with different additives suited to their application.
Where to get steam oil? and what type of oil?
Unfortunately, steam is not usually available from the local service station, though some with but it for you by special order. But any industrial lubricant company who deals with agriculture or other fields should stock it. Another alternative is a large rural or semi-rural supplier (e.g. Elders), who may have 20 litre drums, or buy direct from the oil companies (but there may be 200 litres (44 gal) minimum order). And some ME suppliers have a large drum and will sell smaller amounts to individuals.
[Updated: 10 Apr 2014]
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