i often wonder and doubt studies especially since the API does not rate amsoil. it makes me wonder also how many other independent studies do not include amsoil or ALL motor oil for that matter.
I was under the impression that to get a certification the manufacturer had to apply for it. The API doesn't test any and every oil that hits the market. If Amsoil doesn't have a cert, it's because Amsoil choose not to get it. That doesn't mean the oil doesn't qualify, but why not get it?
As to independent studies, maybe there is a less than honest reason to testing or not testing an oil, or it is justified due to lack of resources or other reasonable criteria. A study up to snuff will explain their reason for selecting oils. The authors may still be selective for reasons not stated, but their bias will be clearer.