What is NOT a recommended practice when replacing gear lube in a differential?

Using old gear lube for refilling the differential is not a recommended practice because old gear lube can contain contaminants and breakdown products that may have developed during use. Over time, gear lube can degrade, losing its protective qualities and becoming less effective at lubricating the gears inside the differential. This can lead to increased wear, potential overheating, and ultimately failure of the differential components. In contrast, proper maintenance involves using fresh, clean gear lube that meets the manufacturer's specifications to ensure optimal performance and longevity of the differential.

The disposal of old gear lube using a licensed recycler is crucial for environmental safety, and draining the old lube into a pan helps to prevent spills and ensures efficient collection for proper disposal or recycling. Filling the differential until the new fluid is at the correct level, just below the fill hole, ensures that it has sufficient lubrication without risking overflow, which can lead to leaks or operational issues. Each of these practices supports the overall maintenance and reliability of the vehicle, while the use of old lube contradicts these benefits.