# 2 is better.
2/ = 50%. So # 2 filters 50% of particles 6 microns and larger.
20/ =95%. So # 2 filters 95% of particles 19 microns and larger.
The second set of numbers in the ratio is where you would like to see lower numbers.
** Do you have or want to use a Wix? While they are great filters the Purolator Pure One filters better.