Do not over-tighten these euro-style plastic filter caps. The O-ring does the sealing. We're talking around 15 ft-lbs max torque.

I overtightened my old VR6 filter cap once and the old age and oil eventually swelled the cap into the threads of the aluminum housing just to the point where it was impossible to rotate the filter cap without destroying the cap itself.

It took me three hours, basically sawing apart the old filter cap with a dremel, then splitting it along the threads. It was like brain surgery, trying to open the skull without touching the brain (or the aluminum threads in my case) It worked, but it taught me that these sorts of filters are a little dainty, so to speak.