When a question pops up twice in a short space of time, it's usually worthy of a blog.
Outlook when running in cached mode against Exchange has a Sync Issues folder. In here you will find details of any issues that occur when Outlook syncs it's offline cache with Exchange. If there are problems with the offline cache, then these messages can be helpful in identifying these - but under normal conditions most of the messages are unnecessary.
The Sync Issues folder is not subject to normal retention policies applied to a mailbox. This is because the folders are local to the Outlook cache - they are not in the Exchange mailbox at all. So how do you ensure there is no huge build-up of messages?
As the folders are local to Outlook, then the solution needs to come from Outlook. As far as I am aware, there is no setting to configure how Outlook handles these folders, but it is very simple to create an add-in that checks the folders on start-up and deletes any items over a given age. I have created an add-in that does this, and also included a setup project for it that means it can be deployed easily (e.g. by group policy or SCCM). The full solution can be downloaded from http://purgesync.codeplex.com/. The usual disclaimers apply!