It's right at the back underneath, in the vicinity of where the oil cooler lines come out of the oil filter housing. You can probably see it better from underneath, but it's easier to get to from the top in my experience. It is 13mm and like I said it's such a pain to get to. I wouldn't be surprised if many are loose just because it's often easier to loosen something in a tight spot than to tighten it, hence being able to move it a little.

-Rog