In my experience with GM engines (most of my life), they all had some sort of piston slap. Believe it or not, the thinner oils actually helped quiet them down. And I'm not talking thinner at operating temperature, but the thinnest at start-up (I know, the two are often related - unfortunately).
My mother has a 2002 Chev Venture with the 3.4L V6 with about 170,000 miles. I use it every weekend for towing. It always had piston slap even since new (worse now though). I've tried pretty much everything in there, and the quietest oils were Amsoil's 0W30 or 0W20. The 0W20 was very quiet not too long after start-up. I've only used the 0W20 in winter though. The 0W30 works well in the summer, and the two oils are very close in terms of cSt at 40C.
Before going thin, I tried going thicker (like 15W40 syn - and no, not in the winter). It had great cold numbers, with a pour point of down to -51F, but I guess it was too thick to keep them quiet until things got warm. Of course, once fully warmed up, the engine was silent on the thick stuff!