If hydraulic oil is mixed with too many particles, it will jam the oil filters, scratch sealing elements and make elements blocked or worn. However, it's impossible to be free of particles in the course of using and production of hydraulic oil. Now national standards of China control the particles of hydraulic oil to make the "mechanical impurities"<0.05%, while foreign countries mostly measure hydraulic oil with cleanness degree of NAS and ISO.

Its hydraulic system has the following requirements on the cleanness of lubricant: hydraulic system with large gaps and a low pressure: NAS10-12 (approximately equal to ISO 19/16—21/18, permitted number of particles ≥5μ/ml: approximately 5,000-20,000; ≥15μ: approximately 640-2,500), medium and high pressure hydraulic system: NAS 7-9 (approximately equal to ISO 16/13-18/15, permitted number of particles ≥5μ/ml: approximately 640-2,500; ≥15μ: approximately 80-320) sensitivity and servo high pressure hydraulic system: NAS 4-6 (approximately equal to ISO 13/10-15/12, permitted number of particles ≥5μ/ml: approximately 80-320; ≥15μ: approximately 10-40) Currently the hydraulic oil produced with common process in China generally can reach NAS 8-10.