I was using a CPL. It's mostly noticeable on the last image of the sunset because that shot was taken about 20min after sunset so the sky behind me was getting rather dark. However the tops of the rest of the images are not affected, and are not darker.
Circular Polarizer, sorry. I know the acronyms can be confusing at first. I never use any other "on-camera" filters. One of these days I'm going to get a nice Neutral Density filter though. But I want to save up and get the VaryND.