Yes, you use antifreeze/coolant mixed with water to cool your engine. But you may be surprised to learn that this mixture cools only the upper two-thirds of your engine: the valves, cylinder heads, and cylinder walls. The lower one-third of the engine-the crankshaft, main and connecting rod bearings, camshaft (with its bearings), timing gears, and pistons-must be cooled by motor oil, which transfers heat to the crankcase. This heat transfer by the motor oil is crucial to the operation of the engine. Valve parts and pistons can reach temperatures of 600° F or more. If this heat is not dissipated by the oil, it can travel down the connecting rods to the rod bearings, which can soften and melt. If you've ever had a failed bearing, you know it's a costly repair. Your oil can perform this cooling function only if the oil level is high enough and the passages of your engine are clear of deposits that could impede the flow of oil. That's another reason to check your oil level frequently and change your oil regularly.