On Mon, 4 Jan 2021 06:37:24 +0000 (UTC), Cydrome Leader
wrote:

What't the lowest viscosity oil anyone has come across here? It looks like
some of the "spindle" oils get pretty thin. Are they used in a special way?
How low are the clearances those bearings run at?

You're on the right track with spindle oils -- they're the lowest
viscosity oils I'm aware of, other than perhaps special
watch/instrument oils. I've used Mobil Velocite #6 in an air over
hydraulic system where the thinnest available hydraulic oil limited
the speed of the system. Velocite #6 (ISO 10) seems similar in
viscosity to kerosene. I've never handled Velocite #3, which is even
thinner.

The thinnest spindle oils are probably most commonly used in high
speed pre-loaded bearings with oil-mist lubrication.

https://www.mcmaster.com/spindle-oil

--
Ned Simmons