The main reason why hydraulic oil becomes greasy is its anti-oxidation performance is poor and it is exposed to the most factors of oxidation. Hydraulic oil works under a high temperature and a high pressure. With metal as the catalyst, it is prone to polymerization, for the carbon and hydrogen atoms of hydrocarbon molecules will react with oxygen to form colloid or carbon deposition.

Another reason is that its resistance to foaming is too poor and has a high air release value. When it passes the oil pump or flows back, the bubbles generated cannot be eliminated timely, which is circulated and "bursts" (bubbles break) under certain conditions to produce a "diesel effect". The high temperature in some areas makes hydraulic oil oxidized or polymerized to form colloid, as a result of which "corrosive pitting" is found at the surfaces of oil pump blades, face of gear and machine parts of hydraulic system, which has a great harm to the equipment.

The more there are unsaturated hydrocarbons in the lubricant, the easier it will generate colloid and carbon deposition. It is easier for paraffin base lubricant to generate colloid and carbon deposition than lubricants with a naphthenic base or an intermediate base.

High-quality hydraulic oil should be lubricant refined by adding hydrogen. The higher the refining degree of the base oil is, the better its anti-oxidation performance will be and the more difficult for hydraulic oil to generate colloid or carbon deposition.

The performance of common hydraulic oil depends on the base oil to the largest extent. The content of additives is very low, which generally will not exceed 1%. Its stable function is realized by the performance of the base oil.