10. Overheating of a hydraulic system may be a result of __________.

Incorrect fluid viscosity increases friction and leakage, converting energy into heat and causing overheating. Hydraulic oil must maintain the manufacturer's specified viscosity range to ensure proper flow and minimize energy loss. Viscosity that is too high increases friction, while viscosity that is too low increases leakage, both resulting in wasted energy and increased oil temperature. Options related to recirculation, pressure variations, and oil level do not directly cause overheating.