Do I need to restart the Subscriber Portal after changing the syndication interval at syndicator end and rebooting the Syndicator Portal?
No, you do not need to restart the Subscriber Portal if it is a one way syndication. According to the WCM Syndication Design, the SYNDICATOR initiates the syndication. So in the case of one way syndication, you only need to restart the Syndicator. However, in the case of two way syndication you need to restart both servers since both are acting as Syndicators!