And at the same time it will pass a thick oil... all due to surface tension, polarity, etc. Which is to say don't think about the filter as a geometrical sieve

And at the same time it will pass a thick oil... all due to surface tension, polarity, etc. Which is to say don't think about the filter as a geometrical sieve that has holes that pass molecules solely based on their size (either it fits or not). This is not true even at macro level.