Just stumbled upon this thread as my FG XR6T has a weeping rocker cover gasket - I took the rocker cover off to discover basically exactly what the OP discovered ... drivers side timing chain nylon guide broken, the piece lodged snuggly between the timing case and the tensioner, and the camshaft lobes showing excessive wear. 265,000 and the engine is bog stock, driven daily and sweet and quiet - seriously considering just slapping it all back together and pretending I never saw a thing!
There'd have to be thousands of FG's rolling around like this with the owners none the wiser, wouldn't there?