Hello all. Long-time lurker, first-time poster. I have an issue that I think people here could certainly offer educated opinions on. I have a Plymouth Horizon in which I swapped a Chrysler 2.4L DOHC. It's got a big turbo and makes in the neighborhood of 500-550 hp judging by 143mph trap speeds. It typically does not see a large number of road miles, but last year I ran Hot Rod Drag Week with it. With a total of about 1200 miles and maybe 2 dozen drag strip passes on it, I noticed quite a bit of carbon build up on the pistons. I pulled the head off and sent the head out to get checked, and the found it had large amounts of valve guide wear. They are manganese bronze, and the valves are stainless steel. It was an expensive rebuild. The cause was inconclusive and would like to prevent it from happening again. I had been using Mobil 1 5w30 full synthetic. I was thinking that something with a higher zinc content might help. Any opinions and suggestions would be welcome. Thank you.