I found this information from Penriteoil which I thought was very good at explaining or at least helping someone like me understand more about group oils, their designations and a description of that designation. You have to open the pdf which opens up a document called Understanding Engine Oils. Hope this is helpful and interesting for some. Regarding Penriteoil...I am not familiar with this company nor have I seen any of their products in places I shop in Texas. Here is the link Understanding Engine Oils