I do not think that there is any difference. Petroleum engineers normally inject water into an oil reservoir to maintain the reservoir pressure (and hence the ability of the reservoir to pump oil to the surface). In the process and if the water injection wells are properly located, the injected water normally sweeps (pushes out) out more oil effectively flooding the reservoir and increasing the amount of oil that is recovered from the reservoir. This incremental oil will otherwise be left behind in the reservoir. Hence, in an oil reservoir where the natural aquifer is large and strong enough to maintain the reservoir pressure, water injection is unlikely to significantly increase the oil recovery from the reservoir.

Oil level is overfull.

Yes it does have an oil reservoir Pretty much all equipment with hydraulics have a reservoir.

On the oil pan reservoir mounted on the bottom of the engine block.

On the oil pan reservoir mounted on the bottom of the engine block.

• A reservoir drive mechanism is that mechanism in which oil is derived by water or gas or both water and gas drive the oil from the well. Is called reservoir drive mechanism.