Other than a drop in oil pressure, one of the most attention-getting engine issues might be a significant drop in fuel pressure. A reader with a Lycoming IO-540-powered Rockwell has been chasing this unsettling issue where the fuel pressure drops to the lower red line above 6000 feet and lasts up to five seconds.
There is also a slight drop in RPM and fuel flow and this will repeat itself a couple of times and then go away on its own. If he turns on the electric pump (a logical response), the drop in pressure tends to go away until the pump is switched off. He tried different fuel selector configurations—left tank, both and right tank only—with the same results.