Lack of maintenece, high milage on oil, leaky gaskets (water mixed with oil) and sitting for a long time can do that as well as other things..
I even had one brand of oil start gumming things up within 500 miles of a change.

If it is that gummed up and sat for that long,'I' wouldn't rely on quick fixes and engine flushes. Then again, I have never trusted that method (the shortcut method) too much after seeing a 302 get locked up while the shop teacher poured kerosene in the carb while running to burn off carbon.