Sean,
About "too heavy a lubricant being detrimental and increasing drag". The short answer is Yes! If the lubricant is too heavy and not pumpable, i.e. grease, it might not be possible for it to penetrate into the surfaces interface. Depending on the load on the bearing, however, a lighter lubricant might not be capable of supporting that load and we are back to direct contact of surfaces. I think, however, that grease would also be perfectly alright for lubrication in a turntable, but the drag on the drive system might be compromised.
As to the "goal of a lubricant", the main reason for lubricants is to SAVE the mating surfaces from WEAR, not to necessarily save the energy necessary to turn the spindle in the bearing. The best way to reduce the wear on the bearing is to reduce the friction between the surfaces, either by introducing a substance that fills the small microscopic bumps and ridges in both surfaces, effectively "smoothing" the surfaces, or by introducing an intermediate substance between the surfaces, thus protecting the surfaces from wear. In the first case greases, solids (graphite) and oils can be used. In the second case, various fluids are used, including air.
I am not sure about your other query about making the slipperiest substance "known" to man more slippery?
Once two bearing surfaces are protected from wear, the job of lubrication is done. The increased drag etc. is not relevant to the lubrication, but might be important for other reasons.
Back to the original arguement. First, The fact that a turntable turns longer with a particular lubricant in it is no indicator of lubricating "goodness". Second, for the purposes of lubricating our turntables, any motor oil is overkill. As I facetiously said, olive oil might very well serve the purpose of reducing wear in the spindle bearing. You might want, however, to use motor oils because there are many different viscosities available and you might want to play with those to determine if some "sound" better than others.
I shall stick to light "sewing machine" oil for my Oracle Alex III, (13 yrs old) but I am tempted to add graphite to the oil, just to see if I can "hear" the difference. It might even make a long term difference to the life of the bearing.!
Regards, Bob P.