FWIW, the Metro 23 does have a loss of oil px procedure (AFM page 3-9). It says:
LEFT OR RIGHT OIL PRESSURE WARNING LIGHT ON
If low oil pressure is confirmed by the oil pressure gauge, the engine should be shut down by applying the Engine Failure During Flight checklist.
If the oil pressure gauge indicates normal pressure (70 – 120 PSI) monitor engine parameters. However, if any abnormal indications appear (i.e. uncommanded RPM fluctuations, rising oil temperatures, etc.), shut down the engine by applying the Engine Failure In-Flight checklist.
I agree that the above is missing in the M3, so is likely a certification issue as others have noted.
Interestingly, landing the TPE331-11/-12 as installed in the Metro with a
low pressure (around but not below 40 psi) is a bad idea because it's quite possible that the prop pitch will end up somewhere between fine pitch and feather, and with the engine still running this can produce significant thrust on that engine, which creates 'interesting' control problems during the landing roll...
grade-3