Ok guys, I don't want to state the obvious here, but these cars have aluminum heads. They came with stellite seats from the factory, which will run just fine on unleaded, but the lack of octane is the problem. I work on Nissans for a living, and rarely see a Z come in with valvetrain problems. Aside from that, my 72 has 267,000 miles on it and just had the first valve job in Jan of this year. The only thing I've done is recurve the distributor for running 91 octane, which is the best I can get locally. Devin aka 72240Z