We have put in place Filteration of our MT4400 Final drives by Kidney loop every 500hrs service since Sep 2019. The Final Drives oil used to C/O every 2000hrs service but now we can save money with no drain the oil. Trending of ISO cleanliness and wear particle shows all positive but Dirt Contamination level has not changed or even gets worse. Inspection of dirt entry such as breathers has been done with no improvement. How big are Dirt particles? What Filteration system is applicable for dirt contamination control?