I'm not clear on what exactly makes an oil a good "extended" drain oil? TBN plays a big part but what do specs. such as HT/HS and DB 229.3 have to do with it? If you take Amsoil, M1 and Schaeffers, all can go many miles but what makes one more suitable for extended drains then another? I believe all of Amsoil's oils pass the DB 229.3 test, while only 0w-40 M1 does in the Mobil line.