Improvement, in hydraulic systems, such as for vehicle transmissions and the like, in the form of a hydraulic reservoir with integrated heat exchanger having fluid supply/return and fluid return ports, including a central section, having inner and outer shell portions with a first gap therebetween, the outer shell being provided with a plurality of radially-spaced, external cooling fins, longitudinally-directed for the length of the central section; the ends of the latter being closed off via respective bottom and top cap portions, thereby defining a central fluid cavity, the top cap portion having spaced outer and inner cap portions, with a second gap therebetween, with a fluid inlet port, in the outer cap portion, directing incoming fluid from the second gap into and through the entire length of the first gap before entering the central fluid cavity, thereby maximizing heat transfer from the fluid via the plurality of cooling fins.