Final answer:
When changing the oil, the square seal should remain on the old oil filter to prevent potential leaks with the new filter. It's important to inspect the engine block to ensure the seal is not stuck there after removal.
Step-by-step explanation:
When a technician is changing the oil, what should remain on the old filter is the square seal. Sometimes also referred to as an O-ring or gasket, this seal is essential for creating a tight seal between the oil filter and the engine block. When removing the old oil filter, it is important to ensure that the square seal does not stick to the engine block; otherwise, the new oil filter may not seal properly, which could lead to oil leaks. After the filter is removed, the technician should inspect the area where the oil filter was mounted to confirm that the old seal is indeed attached to the removed filter.