The correct answer is B) incorrect fluid viscosity. Overheating in a hydraulic system is often caused by incorrect fluid viscosity. If the viscosity of the hydraulic fluid is too low, it will not provide adequate lubrication, leading to increased friction and heat generation. Conversely, if the fluid viscosity is too high, it will create excessive resistance to flow, also resulting in heat buildup. The other options are incorrect because: A) a high oil level would not directly cause overheating, C) normal load variations should not lead to overheating if the system is functioning properly, and D) slow recirculation alone would not necessarily result in overheating.