Almost any film of liquid will reduce friction between two solids. Look at the fine job the water produced by the pressure of your tires on ice does. Sulfuric acid will lubricate, but we try to keep it in the battery. Aside from being liquid, staying there, not causing other problems, mostly what a liquid needs to do is wet, that is spread out and cling to the surface. So much of our oil selection is about choosing a viscosity that can be pumped to the clearance and have the film strength not to be squeezed out, and the big one, stability.
In the early days, the best oil was certain fractions distilled out of paraffinic based crude oil. Unfortunately the stuff thick enough to protect an engine at operating temperatures, might be too thick to allow starting a cold engine. Today most of our oil comes from naphthenic crude and requires much more refining and processing to produce stable lubricants. Some of the products produced work better than the original paraffins. There are a host of additives that improve the oil too. Still it is mostly having a benign liquid film in the clearance. Unstable liquids won't be there long.
[ October 29, 2003, 12:08 PM: Message edited by: labman ]
1. A fluid that is used as a lubricant must resist both normal-force squeez-out and shearing, such as in a slider bearing (piston/ring/liner) or as in a journal/bearing, which is why one of the qualifications of an oil is that it have viscosity, or a resistance to squeeze-out and shear. Some oils do that better than others.
2. Wetting of the surfaces. This means that an oil has to have the correct surface tension. You don't want a fluid that turns into isolated globules.
3. Protection of the metal from rust and corrosion.
4. Decent cooling ability.
As labman said, additives enhance the base fluid to produce a fully formulated oil. The additives are ususally organic chemistries that mix with oil and provide increased friction reduction, metal protection from rust and corrosion, reduce oxidation of the base oil, extend temperature limits of the base oil, Anti-Wear protection, and in the case of gear lubes, Extreme Pressure protection.
Bob has some additional info here:
[ October 29, 2003, 12:36 PM: Message edited by: MolaKule ]