Having just received my 2nd "new to me" U1 in the last 3 months, I have to state that I'm very impressed with the time keeping accuracy. The U1 has been on my wrist since Monday evening and it's gained only 2 seconds in over 60 hours or +0.8 secs/day. That is just amazing accuracy for a mechanical watch. The first U1 I had was also keeping time at under +1 secs/day.
So the question begs is it the stock ETA movement or is Sinn doing something special to these movements to increase their accuracy. I have heard of the special cold temp oil Sinn uses on the U1/U2 series movement. Is there anything else?
Anyway, I'm very impressed with the U1 series time keeping accuracy.