I'd run Amsoil until Pat started using Rotella synthetic after his BB mod. I tried it a few times, then went back to Amsoil 20W/50 because my valves sounded louder (loud valves save lives ). BUT, this winter, using Amsoil 20/50, at 22°F in the morning, my battery just didn't have the power to dig a trench through that molasses, so I went back to Rotella synthetic.