I have been trying oxipng with my screenshots from June and July to see which option is fast and efficient.
oxipng --zc 11 -f 0,6,7,8 --fast *.png
That's slightly better and faster than -o2 which uses -f 0,1,6,7
Alternatively, you can write:
oxipng -o2 -f 0,6,7,8 *.png