Water can damage oil properties and reduce life of hydraulic machinery by alarming 50%. Due to low tolerance in modern machinery the continuous pressure vacuum conditions cause the moisture to expand and explode on the walls of metal surface causing wear and  continuous blasting of inner surfaces with particles and vacuum explosions.
Corrosion into small microscopic pores will cause fatique and wear of equipment. Water increase oxidation in presence of iron and copper as catalyst and the acidity of oil is increased and with TAN number increases to >1 leads to oil changes expense. Constant changes in temperature leads to condensation and corrosion and addition of contaminants in system.

Oil analysis is important to maintain the mechanical integrity of system and to prevent  damage to components. Oil’s TAN will increase with   time or if exposed to high  temperatures and water presence.The oil becomes oxidised (high temperatures will   cause oil molecules react with the oxygen within the air). Oxidation will severely affect  oil’s ability to save internal components and can also hinder the viscosity.