Amsoil recommends their oils for 15K when using amsoil filters. This is the results of their extensive testing. I know the oil can go longer, with oil analysis, but most people don't want to spend that time and money, they just change it. Sure you could do it with dino oil, but you need to use oil analysis about every 5K to make sure its still usable. Synthetic oil is much more stable and less likely to break down than dino oil. And there is no replacement for high quality filters.