What are the long-term effects of using a hydraulic fluid with a lower-than-specified viscosity?

Using a hydraulic fluid with a lower-than-specified viscosity has several negative long-term effects on a hydraulic system, primarily due to increased internal leakage and reduced lubrication. Viscosity refers to a fluid's resistance to flow; a lower viscosity fluid flows more easily. Hydraulic systems rely on the fluid's viscosity to create a seal between moving parts, such as pistons and cylinder bores, or valve spools and valve bodies. A fluid with insufficient viscosity will leak more readily through thes....