> My Omega Seamaster gains ~2-3 minutes a month (so roughly 4-6 seconds a day.)

That falls within the bounds of accuracy required of a Swiss certified mechanical chronometer (that is to say, a clock accurate enough for marine celestial navigation) and is pretty good.

As a marine engineer I have a genuine and real appreciation of mechanical chronometers, though any working watch/clock I use will always be quartz as for accuracy and price, nothing else comes close.

https://en.wikipedia.org/wiki/Marine_chronometer

For something mechanical operating under random g forces and over a wide temperature range, that accuracy is truly astonishing.

Fully agree, automatic mechanical watch movements are truly a marvel of engineering. The fact that the movement is powered purely by the kinetic forces naturally applied from wearing it, and can reasonably accurately keep time over years, under a wide range of conditions… is truly impressive.