Synthetic oil ratings must follow the same guidelines as conventional oils. If the oil is an API licensed oil, then it conforms to the American Petroleum Institute’s minimum performance tests. That means the oil meets the requirements set forth concerning engine wear, fuel economy, deposits, emissions, and so forth. The API license further allows manufacturers of petroleum based products more flexibility in the interchange of base-stock materials. This is not the case for manufacturers of synthetic oils and lubricants. However, the SAE ratings may differ considerably because synthetic oils have a lower cold flow rating than most conventional oils.
For more info, see Synthetic Oil vs Conventional Motor Oil