I have never done a study on engines that have dropped a valve, but I believe there are 2 common types.
First is it broke at the lower notch of the valve stem where the retainer latches. This can be explained by micro cracking during the manufacturing process. Whether it's inclusion in the material or surface scratching, your guess is as good as mine. This fails catastrophically without warning.
The other type is caused by excessive wear on the stem as witnessed by the recall on GM LS7 engines. This creates a valve train noise that develops over time. Hard to tell and it requires trained ears to detect without opening the engine.
I have experienced a valve dropped on my '82 Toyota Land Cruiser FJ40. Luckily, it happened at a red light while idling. The engine simply stalled and refused to turn over. Opening the valve cover revealed the cause and after a new valve, lapping of only that one valve, and reassembly, the engine was back. A tiny nick on the piston and on the head were all the damages left inside the engine. In a way, it was a lucky engine.
At any rate, I'm glad the car now is running again. Hopefully the oil leak is a simple problem that can be easily fixed.
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