The quantity of oil delivered to the steam engine cylinders of a non-condensing car was not too important as the oil did not return to the boiler.  The only condition to be avoided with a non-condensing car was insufficent oil being fed to the engine.   When the condensing cars were introduced the oil added to the steam and sent to the engine for lubrication returned to the water tank and eventually the boiler.  Thus it became important to limit the oil used to only what was necessary for proper engine lubrication.  The plunger type oil pump was found difficult to adjust to deliver the very small quantity of oil acceptable as a minimum to the engine. Consequently the oil was heavily diluted with kerosene (85% Kerosene to 15% oil according to Atlantic).  Now the pump could deliver oil in larger volumes and only a minimum amount of the oil actual oil would find it’s way back to the boiler.  On the later condenser models such as 740, 745 etc; an entirely redesigned pump with ratcheting type pump was used which permitted very fine adjustment.  With this pump, which is mounted in the oil tank, undiluted oil is used.

Heating raw petroleum collected from oil wells and drawing off the vaporized gasses at different temperatures provides various products such as gasoline, kerosene, and diesel fuels as well as lubricating oils also known as mineral oils. Water will displace most oils, with the exception of animal based oils, and consequently special compounded oils that will lubricate in the presence of water are needed for successful steam engine operation. Modern steam oils contain 4% tallow by volume that comes from animals. Animal oil based tallow is produced by heating or boiling animal carcasses, and collecting the liquid residue. It is this tallow oil that makes steam oil work in the hostile internal environment of the steam engine. In practice the petroleum producers place several compounds in steam oil to help stabilize viscosity and lubricity; hence the name compounded steam oil.  Steam oils are manufactured in several blends depending on the temperature of the steam they are to be used with.

Steam pressure and temperature at the cylinder is the primary consideration in selecting the correct steam cylinder oil.  As a general rule the higher the steam pressure and temperature, the greater the need for a higher viscosity of oil, and the lower the amount of compounding which is required.  If temperatures and pressures are relatively low, then lighter viscosity oils with high percentages of compounding are most effective. Other mineral based oils such as internal combustion engine oil and machine oil must be avoided because they will not be able to maintain their lubricating properties in the presence of hot water and superheated steam.  The result will be metal-to-metal contact between the internal parts of the steam engine assembly, which will result in the scoring and galling of mating wear surfaces.

For additional information on steam cylinder oils see the two Exxon-Mobil web pages listed below;