just for reference the oil used is just as important as the filter.
Sludge is a combination of burnt hydrocarbons mixed in the oil itself. What I'm saying is it's not just the filter. The oil needs to have enough additives to keep the particles in suspension and this allows for the filter to do it's job.
Changing your oil more frequently will help even though using sythetics they say 6K is good. I try for 3k-4K and use Mobil 1. Seems to do one of the best jobs at keeping things clean.
Hope this helps.