[OliveMate]

I've been stuck down the Casio modding rabbit hole as of late. I knew filling the watch with oil ('hydro-mod') lead to a crisper display with better viewing angles and increased water resistance, but to see a watch with minor splash resistance operate as such depths is insane.

Worth mentioning some drawbacks before you get your precision screwdrivers out. Doing it will make your watch get stupidly hot in the sun, the process can be messy, and sometimes certain mechanisms/features can break as a result of it. Best to check what others have done before you.

[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.