My $02 is to not use full synthetics but many do. While synthetics do act "thiner" in motors and have been know to reveal leaks in higher milage engines through seals I suspect your leak is due to reasons stated above.
IMHO I would use the "semi" synthetic factory fill 0-20. Walmart carries the Motorcraft in 5qt jugs for $14
The 09's (4 cyl anyways) has a filter "insert" element which for now I can only get at the dealer vs the "screw" on type standard filters. It includes a new "O" ring for the filter case and drain plug on the case. | 
