Which procedure is necessary before adding oil to ensure it is free from contaminants?

Before adding new oil, it is essential to drain the sump to ensure that the system is free from any old oil and contaminants. Residual oil that remains in the sump can mix with the new oil, leading to potential contamination and reduced efficacy of the lubrication system. Draining the sump removes any degraded oil and particulates that may have accumulated, thus allowing for a fresh start with the new oil and ensuring optimal performance.

The other choices, while they pertain to maintenance practices, do not directly address the specific need to remove old oil before adding new oil. Changing the filter is an important part of maintenance, but it doesn't eliminate the old oil that could contaminate the new oil. Clearing debris doesn’t necessarily relate to the action of oil replacement. Lastly, removing the dipstick is primarily for checking oil levels rather than preparing for adding new oil. Thus, draining the sump is the most critical step to maintain the integrity of the lubrication system.