Combine 6 cups water, rice, and lentils in a large bowl. Cover and let stand 8 hours. Drain and rinse.
Place rice mixture, 1 1/4 cups water, and sugar in a food processor; puree until smooth (about 1 minute).
Spoon batter into a clean bowl. Cover and let stand in a warm place (85°), free from drafts, 12 hours or overnight. Stir in salt.
Heat 1/2 teaspoon oil in a medium cast-iron skillet over medium heat. Spoon about 1/3 cup batter into pan. Turn pancake when top is covered with bubbles and edges look cooked (about 1 minute). Repeat with remaining oil and batter.