Any disadvantages to running a vaccuum pump? (for oil)

Food for thought, but more expensive gauges can read what's called 'absolute pressure', which won't change with vac or pressure in the crankcase. It will measure only the 'deadhead' pressure at the sending port. But these types of gauges tend to be far spendier (the measuring device has a vacuum chamber inside it which requires calibration), so I'm not sure many oil pressure gauges measure absolute. Instead most measure 'gauge' pressure, which is the difference between atmospheric and the referenced port.

Some folks may actually have absolute oil pressure gauges, but the types of people to use them will likely know exactly what they're measuring.