This is an interesting and informative thread. I don't have a lot of experience or understanding of just how a high speed spindle's bearing set should be lubricated. I do know that my model gas turbine engines use ceramic bearings for upwards of 160,000 RPM, and these are lubricated via a total-loss, non-recirculating system that delivers kerosene with about 3% synthetic 2-stroke oil content.
My 50,000 RPM KaVo spindle is supposedly lubricated for life. It uses 1-bar compressed air to pressurize the spindle body and prevent foreign substances from getting into the bearings, but as to the nature of the lubrication for them, I have no idea. The user manual for the spindle does not mention anything like an in-line oiler.
Diesel, that may be something you'd want to consider for your spindle project. It'd be relatively easy to drill and tap the spindle body for a pneumatic fitting, and into that fitting, you could create an oil delivery system. At the very least, filtered overpressure in the spindle body should help protect the bearings, as well as keep them cool.
Given that, a light synthetic oil would work well. Gravity as well as air pressure, and centrifugal forces, could deliver the oil where it is needed. It'd probably be one of those cases where less oil is better, just a very slight weeping of light oil when the spindle body is pressurized.