I ran the Motorcraft semi-synth for my first three oil changes at 956 miles, 6,630 miles, and then at 10,680 miles with a motorcraft oil filter. I switched to Amsoil signature series full synthetic at 15,472 miles. Still using motorcraft oil filters. I'm hoping the Amsoil will decrease buildup of carbon deposits on the valves. I plan on pulling the intake to check out my valves with a boroscope at 20K.