This answer and this answer address the question Do reaction wheels generally run in vacuum, or are they pressurized? Sealed or vented? and they both mention that sometimes momentum wheels are sealed with a low pressure (inert) gas rather than vacuum.
edit: I found the following in Sinclair et al. SSC07-X-3, 21st Annual AIAA/USU Conference on Small Satellites:
Many small satellite wheels use a hermetic housing filled with a low-pressure gas. The internal atmosphere allows the use of bearing lubricants with modest vapour pressures as well as protecting parts from contamination or damage.
Gas would create additional friction for the system in addition to bearing friction (and possibly eddy currents), which means additional low power. It also means if power to the motor is not available for a period of time, angular moment would couple back to the spacecraft more quickly.
So there must be some benefit to the presence of gas in some applications.
This answer says:
There's a problem with liquid lubricants: They evaporate in vacuum. For this reason, most reaction wheel assemblies are hermitically sealed with a low pressure inert gas inside the container.
But my understanding is that evaporation is really more a function of the partial pressure of the material in question within the gas, not the total pressure of the ambient inert gas. I don't understand how the gas could inhibit evaporation.
So I don't understand why some reaction wheels would be sealed with a low pressure gas, and others with vacuum. What are the trade-offs and why is vacuum chosen in some cases and low pressure gas chosen in others?
The table below (open in new window for full size) is from Lubrication of Attitude Control Systems and was originally linked here. It's a helpful review of the subject, and the vapor pressures of various lubricants (some more vacuum-friendly than others) are listed near the bottom row. It's worth reading the PDF, it's a nice presentation of the issues.
The right hand side of the table is cut off in the original PDF.