there are many reasons on sludge formation, two of most common are not changing your oil base on required PMS (preventive maintenance service) 2nd is oil contamination, probably by water or a blow by. a multi-grade oil like 20W50 or 10W40 has no difference when use in our country compare to countries with winter seasons.

i suggest a thorough engine flushing would eliminate the sludge before changing the oil.