As any WSUS admin knows, the sync history for some unknown reason is not an option for cleanup using the built in cleanup wizard. This can result in excruciating load times when trying to view the sync history. Hopefully Microsoft will correct this in future releases however until then you can execute the following query against your WSUS DB to cleanup the Sync events.
DELETE FROM [SUSDB].[dbo].[tbEventInstance]
WHERE [EventID]=381 or [EventID]=382 or [EventID]=384 or [EventID]=386 or [EventID]=387
You may consider running this as a part of your monthly maintenance.