Also, adding a small amount of flake graphite (It is conductive.) to whatever oil is used will help with spindle grounding because most bearing assemblies won't provide that ground without help. I suspect that is the sole reason that Linn specifies "black oil" for use in their turntables.
What grade flake graphite do you recommend?
What kind of bearings will benefit from the grounding properties of flake graphite? For instance, some turntables have a metal ball at the bottom of the bearing spindle that rides on a metal thrust washer which is housed in a metal bearing well. I would not think that this type of bearing would benefit from having graphite in the oil. It should be well grounded due to the contact between the metal ball and thrust washer. Are you talking about bearings that have either a non-metallic ball or a non-metallic thrust washer or both?
If the graphite has a higher specific gravity than the oil it is in, it will eventually sink to the bottom of the bearing well, but aren't you concerned about the flakes that get caught between the bearing spindle and the bearing sleeves? In a turntable bearing, which may have clearances a few thousandths, it seems like there could be some building up of graphite between the bearing spindle and the bearing sleeves which could cause it to bind (depending on the finish and clearances of the bearing).
Just thinking out loud here. I would love to hear your thoughts.