Part of a properly completed purge procedure is to turn the engine over by hand until an oil pressure reading is attained so you shouldn't be starting the engine after this type of maint. and being surprised by there being no oil pressure.
With the D-180, the oil pressure sensor is fed its reference voltage from the aircraft electrical system so if you somehow started the engine but then had the master switch turned off with the screen operating on its backup battery, the oil pressure would read zero
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RV-6A (aka "Junkyard Special ")