A spool valve is provided with a stem portion that defines a fluid passageway formed by either (a) a single central support having a non-cylindrical curved surface shaped hydrodynamically, or (b) only a pair of sidewalls with, preferably, interior surfaces that are also shaped hydrodynamically. These stem passageways, which are designed to facilitate the flow of high-speed/high-pressure fluids controlled by the valve, are maintained in a predetermined orientation relative to the ports of the valve cylinders by a mechanism preferably including (a) a cam-following roller supported in a tang fixed to each spool and (b) a two-element cam that captures the roller within two of the parallel sides of a cam-track groove formed on the respective interior surfaces of each cam element.