On one hand, the term "syncretic" makes me think of an organic process in which two traditions merge out of either mutual (or similar) beliefs... or when two distinct cultures live harmoniously for a long period of time and eventually use some of the same terminology, whether or not they hold the same meaning for both groups.
Manichaeism, for me, doesn't completely fit into either of those categories, as Mani seems to have hand-crafted his religion in order to seem cohesive with the major religious traditions in his part of the world. By redefining terms such as "Buddha" and "Bodhisattva," as well as using his own concepts of Light and Dark to explain the origins of mankind ["Eve, whose body contained less Light than his, allowed herself to be seduced by a demon, whereupon she gave birth to Cain and Abel." (Klimkeit article)], he in a sense was able to provide a tradition which all people could instantly relate to, whether or not he was using their religious terminology "correctly." I have difficulty calling Manichaeism a "syncretic" tradition with the knowledge of its goal-oriented formulation; it seems to have been too calculated, focusing solely on targeting specific religious traditions and leadings their followers toward his religion instead.
But maybe I'm being naive in my idea of what a truly "syncretic" tradition should be!