Oil pump hell.

Back to the original problem..... OP, yes, the pressure relief valve could be an issue. The pumping of oil when in the vat may occur even with an open pressure relief valve since you don't have any back pressure with the pump out of the engine, but once in the engine with some back pressure, an open relief valve would come into play.

The filter only has a bypass valve that opens and allows oil to bypass the filter element when pressure drop in the filter element exceeds approximately 8-10 psi.

Is your drill running at around 400-500 rpm? And you ARE turning CW, right? (I suspect you are but...) When the pump pressure builds in the engine, then the drill will load down noticeably.