It is the AFM engines. Does it a lot more on the highway drivers when half the engine is shut down.
The piston slap is a non issue. My WS6 does it, my 98 Z28 did for 156,000 miles, with over 500 12.2 1/4 mile passes on a dry 100 shot of nitrous and it never used any oil. My old 02 6.0 slapped never used a drop of oil. Surprisingly the Caprice is really quiet, but I have AFM removed, it didn't use any oil either way.
The 6.0 from those years is not involved because no AFM, no oil burning issues.
Tons of 300,000 mile plus 4.8/5.3/6.0/LSx (ls1, ls2, ls3 etc) running around with piston slap.