What tool should be used to check oil pressure before replacing the sending unit?

Using a mechanical oil pressure gauge is the correct choice for checking oil pressure prior to replacing the sending unit. This gauge directly measures the oil pressure in the engine, providing an accurate reading of the oil pump's performance and the overall condition of the oiling system.

A mechanical oil pressure gauge is typically connected directly to the oil pressure port, allowing for real-time measurements which are crucial in determining whether the oil pressure is within the manufacturer's specifications. This step is important because if the oil pressure is truly low, replacing the sending unit without confirming the problem may not resolve the underlying issue, which could be related to oil pump failure, clogged oil passages, or other engine problems.

The other tools listed serve different functions. A digital voltmeter measures voltage in electrical circuits, making it unsuitable for assessing oil pressure. A vacuum gauge is used to check vacuum levels in the intake manifold and diagnose issues related to air intake, rather than oil pressure. A compression tester is specifically designed to measure the compression levels within each cylinder of the engine, providing insight into the engine's mechanical condition but not related to oil pressure measurement. Therefore, for verifying oil pressure, the mechanical oil pressure gauge is the appropriate tool to use.