On most of the Little Reciprocating Engines you see on eBay find out if the seller has a suitable Displacement Lubricator for the steam side of the engine.  On engines that will be using compressed air squirt a little Ten Weight non detergent oil in the line as a displacement lubricator will not work without steam.  Those who wish may put a valve between the steam line to the engine and the displacement lubricator.  This way you may isolate your lubricator and drain the condensate and refill with Steam Cylinder Oil without shutting down your engine.  You will note on engines if a slight squeak or moan persists increase your oil to the valves and cylinder.  We have seen bluing of side rods on table top through One Inch to Five Inch Scale engines from heat due to lack of lubrication.  We also recommend a Lighter Grade Cylinder Oil which is about 145 to 165 weigh oil.  It is an old time blend worth trying and we carry it on shelf by the quart.  We do not recommend 6oo weight gear oil.  If you use this it will carburize even at low steam pressures and temperatures and gum everything up.  With a little care and practice you can dial in your displacement lubricator.
Whistle & Bell's!
Regards,
akrailways