In order to meet the increasingly tough emission regulations on diesel vehicles, automobile manufacturers in Japan are focusing on the development of diesel particulate filters (DPFs), nitrogen-oxide-reducing catalysts so-called deNOx catalysts, and other technology for reducing exhaust gases. In order to reduce catalyst poisoning, diesel fuel with a sulfur content of 50 ppm or lower-one-tenth the previous level-was put on the market in Japan in April 2003. At the same time, guidelines took effect for the new DH-2 and DL-1 standards for diesel engine oils compatible with after-treatment devices; the full regulations are scheduled to come into force in 2005. The newly developed low-ash diesel engine oil described here contains a reduced amount of metallic detergents, thus lowering the sulfated ash content to about two-thirds that of conventional high-ash oil. The results of the tests specified by DH-2 confirm that the developed oil is similar or superior to high-ash oils in all of main performances (detergency, wear control, base number retention, and metal corrosion). Through tests on engines equipped with DPFs, the developed oil has also been confirmed to be compatible with DPFs, and the oil's good long-drain performance has been confirmed in a year-long fleet test.