I'm doing some maintenance on my wife's car, 2009 S80, and I've come across a small but stubborn issue. First let me say that this is the first Volvo I've ever worked on. In fact, it's the first euro vehicle.

I've unscrewed this metal cap, thinking I would find an oil filter. The oil filter is actually accessed from the top (I know now). But in the mean time, I can't get this sucker back on. What's the trick and what is this exactly?

Thanks in advance!