Run History should be regularly cleared to keep your database file sizes under control. There’s also not a lot of point keeping weeks (let alone months or years) worth of run history in the Sync Service. It shows when profiles ran and whether there were any errors, but click on a CS object and you will only see it’s current state. The interesting data about what changed and when is in the import and export logs, but we need to do a bit of extra work to keep them.
First you need to set up all your Import and Export run profiles to drop a log file. Then you run a script (here’s mine) after every import or export to archive the contents of the log.
These log files are invaluable when tracking down when and how something was changed. I can’t count the number of times the Sync service has been blamed for spontaneously changing things, when inspection of these log files shows it was just responding to data changes imported from a connected system.