I read, in owners manual, 5.3l is 87 octane or better.it also said to only use top tier gas.I know for a fact my 2018 5.3l gets better gas mileage and runs smoother on high octane.as the owners manual suggest 87 octane OR BETTER!!!that OR BETTER is more better.it’s no placebo.so either longer detonation or something is in high octane that makes an engine run better. V8 engine.