My guess would be that in addition to the viscosity, it preemptively compensates for the thermal expansion of the oil, since on your wrist will likely be warmer than room temperature.
Here is another detailed liquid-filled watch guide [0], which talks more about handling thermal expansion, where rather than using an air bubble, he opts for some sort of foam ring. Another person recommends Flourinert with foam rather than mineral oil [1].
Here is another detailed liquid-filled watch guide [0], which talks more about handling thermal expansion, where rather than using an air bubble, he opts for some sort of foam ring. Another person recommends Flourinert with foam rather than mineral oil [1].
[0]: https://www.christopherwardforum.com/viewtopic.php?f=5&t=496...
[1]: https://forums.watchuseek.com/f21/liquid-filled-citizen-6540...