What step is necessary when preparing an engine for indefinite storage?

To prepare an engine for indefinite storage, replacing the oil with a corrosion preventative mixture and running the engine at normal operating temperatures for one hour is crucial. This process serves several important functions.

Firstly, the use of a corrosion preventative oil helps create a protective barrier within the engine, safeguarding internal components from moisture and contaminants that can lead to rust and corrosion over time. Running the engine at normal operating temperatures ensures that this mixture is thoroughly circulated throughout the lubrication system, reaching all critical parts and coating them adequately.

Additionally, running the engine allows for the combustion chambers to be cleared of any fuel residues and helps in the oil's ability to prevent corrosion by effectively redistributing it to areas where it’s needed most. This proactive approach is essential in ensuring that when the engine is ready to be put back into operation, it will perform reliably without issues caused by prolonged inactivity.

While other options might suggest some level of preparation, they may either lack effectiveness in preventing corrosion or do not involve adequate retention of lubrication within the engine system.