Heat oven to 350° F. Brush an 8½-by-4½-inch loaf pan with oil and dust with flour, tapping out the excess. In a large bowl, whisk together the flour, baking powder, and salt; set aside.
In a medium bowl, whisk together the oil, sugar, buttermilk, eggs, lemon zest and juice, and vanilla. Add to the dry ingredients and whisk to combine.
Transfer the batter to the prepared pan and bake until a toothpick inserted in the center comes out clean, 65 to 75 minutes. Cool in the pan for 15 minutes, then transfer to a rack to cool completely.
Slice the cake. Serve with the ice cream and sprinkle with the pistachios.