The GM 2004R uses what is known as a variable displacement pump, or what I like to refer to as a supply and demand system,which varies pump displacement based on the hydraulic systems operating requirements beyond the pump outlet. This is of course, beyond the influence of the pressure regulator valve in the system. Every revolution of the pump moves a certain volume of oil depending on the position of the slide. During upshifts, the oil volume demands of the oncoming friction element increase as that circuit is charged. When this is sensed by the oil pump, the slide responds by positioning itself in a manner that increases the amount of oil volume per revolution to satisfy the demands. This results in reductions in efficiency losses encountered when a fixed displacement oil pump and similar pressure regulation valve and spring system is in use. Although all unused oil is recirculated to the sump with both systems, less is moved under low demand operation due the ability to control volume or output per revolution within a variable displacement system. This is where the improvement in efficiency actually occurs. Now with that being stated, the higher the oil pump spins, the less time the system has to respond with increased volume by the re-positioning of the slide during a high rpm shift when the oncoming friction element is taking a "gulp" of output oil. If the pump slide springs are not matched to the projected operational rpm of the oil pump, the size of the boost valve diameter in use, oil filter and pick up and the list goes on the greater the pressure drop that will occur during high rpm upshifts ,and this is definitely a cause of concern due to the reduction in friction element capacity incurred as the pressure drop takes place. Also it causes the slide to bounce between its stops which occurs during changes in demand cycling of the slide and this often results in sudden slide fractures/failures. Properly calibrated springs keep pump volume high during desperate times reducing slide cycling, and allows the excess oil volume to be handled by the pressure regulators version of "return to sump" evacuation. All this means the slide goes nuts with stock parts beyond 5000 rpm. Install a pressure gage and see for yourself. There are huge difference with just nothing more than a filter change.