I made the decision to use Amsoil 5w40 in my 02 Honda S2000. Due to a dirty looking valvecover I ran some short cleaning intervals before switching to Amsoil. I gotta say there were lots of people questioning this viscosity over the 10w30. Initial impressions are that the motor is extremely quiet at idle now when fully warm, much less valvetrain noise. I can't detect any increased sluggishness either, though I expected to. I'd almost go out on a limb and say the car feels stronger with this oil than previous 30 weight runs. Now I'm comparing a full synthetic to previous conventional and group 3 runs, so I don't know if that has anything to do with frictional levels. I'm going to monitor the fuel economy over the next while, but so far this oil feels great in the engine. It could likely be losing a few hp, but it isn't anything that is detected by the butt dyno. Looking to keeping this car running strong for as long as possible, I think this oil will help in that goal. I'll get some used oil analysis as soon as I can. thanks for all of the advice on this in past posts.