Should the postdoc interview focus on my research findings or transferrable skills that could be value of the research team that I am applying?
Actually it doesn't need to be either-or. Make it both. But don't do it in two parts. Weave it together if at all possible. 

But another reason this is happening is that they want to know something about you as a person and as a potential colleague. How do you present yourself. Are you formal? Pedantic? Friendly? Lots of things. Along with that, they want some idea about how you go about handling such public talks. How do you present ideas whatever they are. If you seem natural it will probably help. Unfortunately that normally comes with a lot of practice over time. But they probably understand that.