Sensors that report pressure, temperature, cylinder position, flow and fluid condition, plus the amplifier and controller cards that drive proportional and servo valves. Five of the six measure something; the amplifier cards are the exception, since they act on the system rather than observing it.

To control it in closed loop, to protect it before damage occurs, and to see condition degrading rather than discovering it at failure. An uninstrumented circuit works until it does not, and gives no warning in between - which on a production machine is the expensive way to run.

Pressure, because it is both the primary control variable and the main safety one, and because so many faults show up as a pressure that is wrong or slow to arrive. Temperature is a close second on any system where oil life and viscosity matter, which is most of them.

Continuity. A sampling regime reports what the oil was like when the sample was taken; an inline particle counter or water sensor reports what it is like now, and flags a filter bypass or a cooler leak within hours rather than at the next scheduled sample.

Yes. Anything in the fluid path must withstand full system pressure plus the pressure spikes a hydraulic circuit generates on valve closure, which can far exceed nominal working pressure. A process-industry sensor of the same nominal rating is often not suitable for hydraulic service.

Commonly as 4-20 mA or 0-10 V analogue signals, and increasingly over CAN bus on mobile machinery or over industrial Ethernet on fixed plant. The choice usually follows the existing controller rather than the sensor, and most sensors are available in several output variants.

Generally yes for pressure, temperature and condition sensors, which need a port and a signal route. Cylinder position sensing is the exception, since integrated transducers require a cylinder built or modified for them - external designs exist precisely to make retrofit practical.

On mobile machinery, high ingress protection plus vibration and temperature ratings matched to the installation, and often EMC compliance for the vehicle. Fixed industrial installations are less demanding, but wash-down environments need attention to the connector as much as to the sensor body.

Periodically, and particularly where a reading is used for safety interlocking or for commercial measurement. Drift is usually gradual, so a sensor giving a plausible but wrong reading is more dangerous than one that fails outright - which is the argument for scheduled verification rather than replacement on failure.

In principle, but it needs the whole chain: a sensor, a controller, and a proportional or servo valve with its amplifier. Adding a sensor alone gives monitoring, not control. The valve and its electronics are usually the larger part of the cost and the larger part of the engineering.