SAPS, is Sulfated Ash, Phosphorous, and Sulfur. Sulfated Ash percentage is the result of a standardized testing procedure to determine the amount of ash that oil creates when burned, Phosphorous and Sulfur compounds are commonly used in oil as anti-wear additives but high phosphorous and sulfur in oil typically create more ash when burned which can contribute to plugging exhaust particulate filters, and phosphorous is implicated in fouling catalytic material, so to protect emissions systems most API S-rated oils are limited to 800ppm of phosphorous, and because of the push of GM dexos are limited to 1% ash (.9% when Dexos1Gen3 comes into effect), When they started putting DPFs on HD diesels starting with the CJ-4 specification HDEO has been limited to 1% ash. For diesel passenger cars in Europe they've been using mid-SAPS ACEA C sequences with a limit of .8% ash for years, more recently as gasoline has become ultra low sulfur in most of the western world, gasoline cars have switched the these mid SAPS oils because it supposedly helps keep the catalyst healthy longer and contributes less to creating deposits on the back sides of valves in GDI engines also in the coming years gasoline engines will start having particulate filters like diesels, there's a few cars out there that already do.