Got no input on your issue, but the '5w' in those oils are their cold viscosity, so there's no difference in terms of oil grades between those two when cold (apart from the obvious brand/type/quality etc...) so you haven't changed anything, and 60 seconds is definitely not enough to get it up to a temp where it would make a difference.
You should really try a 10w30 or 10w40 (10w whatever, the important part here is the 10w instead of there being a 5w). Seen a couple of posts here of people running the Castrol Supercar 10w60 stuff in their later LSs and said it eliminated engine chatter at every point. I've run a 10w50 in every car I've owned, ones that want 10w30, even with VCT or VVT or new or old and wouldn't hesitate to try it on an LS3 if I owned one. I'd even try a 15w/x to see if it made a difference, at least in the short term if that did nothing, particularly in the summer. At some point though there is something wrong if using stupidly thick oil (like 15w or 20w+ on an engine that's recommended for 5w) is the only way to get it quiet.
Have a look at the charts below, left hand side of those curves is the starting temp. Second chart is with the same hot temp viscosity (40 weight) but different cold viscosities (5w, 10w, and 15w). The oil will be thicker when cold and thins out as it heats up which is why they approach each other and group together when reaching operating temp. It's why it's not as much of an issue to run a 10w50 in a car that recommends 10w30/40.
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