[cenamus]

Why does it heat up in the sun?

[Cerium]

The back will get hot since the oil improves the heat transfer from the front to the back. The sun will always heat the front, but as long as the heat transfer rate to the back is low enough it won't feel hot - your body will absorb the heat and reach an equilibrium temperature which feels natural.

[two_handfuls]

My guess: higher thermal mass, so over time it can accumulate more heat than a non-filled watch.

[iancmceachern]

I expect it has more to do with thermal conductivity