Some engine builders historyThis story begins long ago, when overhead valves were first fully enclosed and lubricated by circulating engine oil. A single piston, rising and falling, compresses and expands the air and oil vapor below it in the crankcase. There is no loss in this simple process for the energy required to compress the gas is recovered as it expands on the piston's upstroke.
Adding rocker- or cam-boxes to the head complicated this. Oil pumped to the valvetrain must somehow return to the crankcase - normally left to gravity via a camchain gallery, pushrod tubes or a dedicated passage. Now when the piston descended, the rising pressure in the crankcase rushed up these new passages and into whatever volume existed in the head. Because the passages were small, the first problem noted was failure of the oil to drain back at certain engine speeds, Careful dyno work would also have revealed a power loss, for it takes power to force a "fluid" - crankcase air, in this case-back and forth through small passages.