Sulfated ash is an indicator of how an oil will affect valve and piston deposits as well as oil consumption. A sulfated ash limit of 1.0 mass percent was historically suggested as a maximum for optimum valve and piston deposit and oil consumption control. This was primarily driven by Detroit Diesel 2-Cycle Engines that experienced problems with oils that had greater than 1% ash. That issue has gone away and most heavy duty engine oils are about 1.3% sulfated ash. Their additive packages are driving this higher ash level. Basically, the higher the TBN (total base number), the higher the ash level. TBN is important to heavy duty diesel engines, especially now, because of the sulfur in the fuel (albeit reduced from earlier levels) and the growing use of extended drains of 30K, 40K, 50K, or even 70K miles and more. None of this really comes into play with a passenger car motor oil but for reference, the sulfated ash of a typical passenger car motor oil is anout 0.7-0.9%.