What do Huber oil names mean, e.g. SilOil M40.165/220.10?

SilOil is short for silicone oil.

The first part of the code is the lower temperature limit (°C) of the oil.  This is the temperature of the fluid where the viscosity has reached 50 centistokes (cSt; equivalent to mm²/s); cooler than this and the oil would be too viscous to pump.  The ‘M’ or ‘P’ before the number refer to minus or plus respectively.

The second part of the code (in between two full stops) is the higher temperature limit (°C) of the oil. If there are two figures separated by a ‘/’, the first corresponds to the maximum in an open bath unit (e.g. Ministat) and the second is the maximum in a Unistat. The maximum recommended temperature for a bath unit is below the oil’s flash point, to ensure safety. Unistats are hermetically sealed, so can be used at slightly higher temperatures.

The final part of the code is the viscosity at 25°C (around room temperature) in centistokes, cSt (i.e. mm²/s).  The higher the number, the higher the viscosity.