You're right, WD40 is not a lubrication oil, it is primarily a solvent with a small amount of lubrication properties.
3-IN-1 Oil, however, is a lubrication oil with a small amount of solvent properties (it is also classified as "sewing machine oils"). It is about a 10 weight oil. It will not evaporate any more than any other motor oil, what does evaporate is the little bit of solvent material in the oil, leaving behind the lubrication oil component.
It is great for small motors such as fans in air conditioners. It's low viscosity allows it to flow easily into the tight clearances of bronze bushings (bearings) used in these types of motors. Its low amount of solvent properties allow it to penetrate gummed up bushings in these motors and get the lubrication component into the bearing.
They also make 3-IN-1 Motor Oil. Its basically the same as 3-IN-1 Oil except it is about a 20 weight oil.
(I'm not an oil or air conditioner expert, just an engineer and mechanic