I have an '02 Z28 with 17,500 mi. I changed the oil using German Castrol (was using M1 10W30) and now I have a distinct clattering/diesel sound from 1000 - 1500 rpms. I assume it's the infamous piston slap I've read about. It's never made this noise before, and it doesn't go away when it warms up. I'm going to drain the GC and refill with M1 and see if the noise goes away. Discuss.