Great question. My answer is
Great question. My answer is this. Oil changes depend on the driving habits of the owners. The best way to get the most out of your engine oil is to keep it in the optimal heat range it is designed for. In other words long steady drives. The reason Toyota has 10k intervals is that they have data supporting that this should be the average expected time for change on 0w20. The correct way to monitor oil life is to have it tested in a lab, no one does that though so we have to put a "timestamp" on the interval. The reason Toyota will not change that to a 5k interval for 0w20 is that would ruin the CAFE rating of Prius and they would be hit with a lawsuit. Good question!