6-8 seconds a day is a lot. My recently serviced 3940 does much better. Not knowing service history on used Patek would bother me a bit unless it had recently been to Patek for service. 3-6 seconds is more in spec, it should never run slow, that tells you it needs service. Not sure if Patek could tell you from the serial number when and if they ever serviced the watch but you could ask dealer to check and if so get proof from Patek. Most cared for Pateks would come with all original stuff and paperwork, does this one? If you're set on the 3940, it might be worth waiting for right one, paper work, service history and recently serviced. They did make a lot of them for a grand complication. Also make sure you're happy with a 36mm watch, it is a gorgeous, historic watch but at 36mm the sub dials can be hard to read as your eyes get older, seeing the date is important to me. Regards. Ned.
