Comparisons between a motorcar engine and a turntable main bearing should not be made as they share very little in common. In a motorcar engine, there are no bearings that are constructed in the same way as that of the majority of turnable bearings. In addition surface velocity of moving parts is high (certainly compared to a turntable) and all parts are subject to significant heat cycling. Finally, and most importantly, the oil in a car engine is actively pressurised. In comparison, most turntables use, what is essentially, the same kind of bearing; that is a shaft running in a journal (to support lateral load) and a point (or, more accurately, a radiused surface) bearing against a thrust pad (metal, jewel, or Teflon etc.) to support vertical load. Surface velocity of the surface of the rotating shaft is very low - lower than the vast majority of bearings, whilst surface velocity of the point contact element is extremely low. The load on the lateral load supporting element varies from one turntable to another depending on the weight of the platter and how well balanced the platter is, together with how perpendicular the bearing element is relative to the platter mass (also, the geometry of that supported mass relative to the bearing). In comparison, the vertical load is very much higher than that of the lateral load; the lateral load sees a relatively high surface area, whilst the vertical forces are supported by a very small area of contact (much smaller than any bearing surface in a motorcar engine for example). Despite the high pressure/low surface area contact of the vertical support, turntable oil is not actively pressurised and relies on atmospheric pressure. In many ways, it is more appropriate to think of a turntable bearing as two separate elements.
As mentioned earlier, in a previous post, a gap must exist between bearing surfaces for the lubrication (usually oil) to exist within; the smaller the gap, the thiner the viscosity of the oil needs to be and the tighter the manufacturing tolerances of said bearing need to be. In addition, whilst the bearing is not susceptible to heat cycling in the same way as a motorcar engine it is still susceptible to unequal expansion of the shaft and sleeve elements. In very low gap bearings this can be a real issue and such bearings often have both elements made from the same metal, or materials with very similar expansion coefficients (the Spiral Groove is an example of the former) and in the case of Brinkmann the temperature of the bearing is actively controlled for just these reasons.
There are a number of issues regarding turntable oil; one is that it couples the bearing in the lateral plane - how tight this coupling is depends on the relative viscosity of the oil compared to the relative size of the gap between the bearing surfaces. In addition, two further issues must be considered; one is the shear forces acting within the oil and the other is how the oil adheres to the bearing surfaces - these are independent, but closely related, aspects. Shear forces in the oil (this assumes the oil remains in contact with the bearing surfaces) creates drag, this is not the same as friction as that implies surface to surface contact. This drag acts directly on the rotating inertia of the platter, the tension of the platter relative to the motor and the load applied to the motor - these aspects should not be underrated and can have a significant impact on motor dynamics (more so than electrical power supplies, for example).
Considering the vertical support aspect; bear in mind the oil is only under atmospheric pressure and that the effective pressure of the the bearing elements, at the actual point of contact, is significantly higher than atmospheric pressure. As a result, at the actual point (or area) of contact, no actual oil exists between the surfaces. One should also consider the construction of turntable bearings. In essence, when one considers the vertical support element, there are two surfaces turning (grinding) against each other (albeit slowly) - over time, the harder of the two materials will wear the softer of the two (such can often be seen when one is an extremely hard material like ceramic) - this will only be at the contact point, which is very small. How much of an issue this is depends on the specific materials used and the weight the bearing supports - the actual oil used does not make much, if any, difference to this aspect.
Some synthetic polymer materials used for bearing liners are self lubricating but are also hygroscopic and this means that they are not dimensionally stable - this can make a big difference with those bearings.
Some bearings use Oilite sleeves (or similar), Such materials have lubricants embedded within them. However, if you speak to the manufactures of such materials they will tell you that the operating principle is thus; high surface velocities create friction, this friction generates heat, the heat draws the lubricant to the surface of the bearing liner and lubricates the surfaces. If velocity between the components is very low (as it is in a turntable), insufficient heat is generated and the material does not lubricate as intended. In addition, such bearings are not intended to be used in an oil bath.
In summary, the specific viscosity f the lubricant used can make a significant difference to the turntable; both because of the way it mechanically couples the platter to the chassis and in the way it loads the drive system. By the same token, how much oils there is in ones turntable bearing can also make a significant difference. Many bearings use two bushings, one at the bottom of the shaft and one at the top. If insufficient oil is used, the top bushing is effectively run dry, or partially dry; this will make quite a big difference to how much drag the bearing applies.
Initially, it may seem counterintuitive but a very free spinning bearing is not necessarily a good thing with regard to turntable bearings.
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Sorry, rather an epic!!
