Thanks yes I think Flourinert is one I saw mentioned too, though I am not sure it is available in Japan. According to one of the linked posts, Sinn used to use silicon oil but changed to flourinert (or similar) to prevent dial discoloration.

I think I failed as my silicon oil was just too viscous. The same post notes that the Ronda movement (the same as mine) actually has more torque than the ETA Sinn used in their hydro watches. And that experimenter got his to work. So it must be possible. But no quartz analogue watch has an excess of torque - it would just needlessly waste power - so viscosity is likely my culprit.

In a fleeting spark of executive function, I have decided not to doggedly follow my trail of incompetence further on this one. Even if I sourced the right oil, I have no way to account for temperature expansion of the oil, other than leaving a bubble. Which I don't really want. So it would pop the crystal out in the summer, while I lounged magnificently by the rock pool. Not a great look.

Also my experience clearing up all the oil is not one I want to go through again for every battery change. A solar powered digital watch might be okay though.

I liked the Victorian risk assessment too