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 ]