Through what do oil drain back to the oil pan?

The mechanism through which oil drains back to the oil pan involves specifically designed features known as drainback holes. These are small openings in the engine's design that allow lubricating oil, after it has circulated through various engine components for lubrication and cooling, to return efficiently to the oil pan.

The function of drainback holes is crucial in ensuring that the oil can flow back to the pan where it is stored and from where it can be pumped back into circulation once again. The design of the engine, including the placement and sizing of these holes, is optimized to facilitate gravity-fed return of the oil.

While oil passages may be involved in transporting oil throughout the engine, and oil reservoirs hold oil prior to circulation, it's the drainback holes that serve the specific purpose of allowing oil to flow back to the oil pan. Return pipes, while they may serve in similar applications in some systems, are not the conventional means for drainback in most engine designs. Thus, the correct answer highlights the role of drainback holes in maintaining effective lubrication in the engine.